Hello

My stock 350 is making that strange noise at idle. It does it since i own this car which is about 8 years now.

Never was able to figure out whats causing this noise but somehow, i learned to live with it, since the car is running fine and the noise never changed, but anyway, i would like to figure out whats causing this.

The noise is only really noticable when lying under or beside the car.

https://youtu.be/ZH_LXvmwPuA?si=2kusxjno5NKu636f

It appears only in neutral and park with engine warmed up. If engine gets started cold, its not appearing, but comes in if the engine is fully warmed up. If put in drive/reverse while idling, the noise goes away. If engine rpm get slightly reved up, the noise goes away. Noise is not appearing while driving under load or coasting. Oil pressure is good, about 40 PSI at idle with 10W-30 with a high amount of ZDDP.

In the video, converter bolts are out to make sure its not converter/trans related. Flexplate bolts are tight.
I also had the accessory belts off, but no change. Seems like its related engine internal and more to the rear of the engine.

Timing chain is a brand new cloyes set.

Ive also tried to stethoscope all over the engine with a vacuum hose in ear, but couldnt find a definite spot where its coming from.


Maybe someone has some thoughts about it. Thanks a lot!

 

What year & model?
Speed of Noise increase/decrease w/ change in RPM?

 

71 350 in a 71 Cutlass, #7 heads, stock head gaskets, stock cam/valve train.

The noise goes away with engine speed just above idle. Its only noticable at warm idle with no load (neutral/park/no converter attached).

maybe it could be something oil-pump/drive shaft/distributor gear related?

 

How close is the starter pinion to the flexplate?

 

Good call what Eric suggested. Check any looseness of that pinion gear. The plunger needs to fully retract the pinion. Pinion shouldn’t be loose, it should fully retract.

 

I will double check the starter pinion, but for what ive seen, it has plenty of clearance to the flexplate.

even if it would not have enough clearance, it wouldnt be engine-temperature related?

 

What i know is, when i have the distributor cap off and move the distributor shaft by hand clockwise/counter clockwise, theres a little play in the cam/distributor gears and the "rattling sound" this play makes in between the gears sound similar to my noise at warm idle. My engine was running for i guess a long time with a wrong no lubrication - hole plug in the left oil galley until i installed the right plug. The gear of my MSD distributor shows nearly no wear, but maybe the cam gear suffered a bit from the missing lubrication. No one knows how long the wrong plug was installed. Timing mark is steady with no jumping around thou.

I can imagine that maybe at slow engine speed, those 2 gears are "rattling", and when increasing engine speed it goes away. But that wouldnt explain why the noise also goes away in gear (rpms are lower in Drive). Just some thoughts.

 

Have you pulled the valve/rocker covers off to inspect the rocker, rocker arm trunnions and rocker cover oil baffles?

 

Yes, a few times since i have the engine for different reasons.

Valve covers are stock and the oil baffles are good, not loose or something.

Rockers/trunions show some wear, and will get replaced soon as well as pushrods. But my noise is not under the valve covers and doesnt sound like a typical "too less pre-load lifter/rocker ticking".

 

Take all of the belts off and start it for a minute.

 

Thanks for the help, i already did this. I let it warm up till the noise appears and then took all belts off to see if the noise is still there. Unfortunately with no success, noise was still there.

 

Ok. The noise sounds like a lot lower frequency than engine or even valve train rotation. Kind of hard to tell for sure from the video. Are you sure it isnt an exhaust leak ?

Do you know what rpm the engine is idling at ?

 

Yeah, its no knocking or ticking, its more a "marbeling" sound. At 0:12-0:17 in the video its hearable very good. But, i know, determining noises from a video is difficult.

I have a slight exhaust leak on my drivers side header, but its only possible to hear when going right in the spot with a vacuum hose "stethoscope". Even, my expierience with exhaust leaks is that they get louder and are more a ticking like a lifter under load/driving/bringing rpms up.

Yes. Engine is warm curb idling at ~700 rpm.

Thanks for your help!

 

I love a mystery
Unless I am listening to the wrong noise it sounds like it is "ticking" at about 175 - 180 times a minute. That's pretty close to 1/4 of the engine rpm if you are at 700 rpm. The cam, distributor and oil pump are all at 1/2 engine speed. There really is nothing I can think of that turns at 1/4 engine rpm.

 

Flexplate issue ?

Listened to the audio here and there for 2 days. Was just thinking torque converter, transmission, related...

Came across this below. Description, location, and precise time/rpm of noise seems to be a match...

Problem was diagnosed and resolved in 6 days. So an actual thread with suggestions and most importantly, the OP posting solution confirmed. Something I wish more threads would do...

https://67-72chevytrucks.com/vboard/...d.php?t=757605

 

Haha i would love not to have this mystery

This "ticking" is more hearable in the video then in reality, the "marbeling" is the noise thats all about.

I read somewhere that a possible piston-slap can cause a marbeling/dull rattling sound at idle and goes away with load/rise of rpms. But since mine doing this only while hot idling, this makes no sense to me, because i would guess in case of a slapping piston, the noise should be appearing more/louder with engine cold, before pistons warm up and expand due the right operating temperature.

Mines the opposite. Quit when cold, "marbeling" at operating temperature

 

nice thread, its always good to read about sucessfully solved problems!

I already checked if its converter/flexplate/trans related, but unfortunately without luck.

i had my transmission and flexplate off not long ago to replace the rear oil galley plug. Flexplate looks good for what i have seen so far and the bolts are torqued to the correct spec with loctite. Converter to flexplate bolts are tight too.

Some years ago, when i installed a new converter, i had installed a aftermarket flexplate (maybe pioneer?) that i had lying around (i dont know why i did that), but i swapped back to the stock flexplate (i prefer the quality of stock parts over aftermarket).

Marbling noise is, and has ever been, the same with both flexplates, stock and aftermarket, so i guess its not (or cannot be) flex plate related.

Even, in the video, converter bolts are out to check if the noise stops when disconnecting converter/trans from the engine. Well, its not.

I also would assume, if it would be something flexplate/converter related like cracked flexplate or a loose bolt, the noise would also appear right after a cold start, just as the crank would turn.

 

Certainly have a mystery.

The "noise" sounds ~1/2 engine speed which makes distributor suspicious. I don't suspect it's the oil pump shaft itself since you said you have good oil pressure.

I don't know what a MSD distributor shaft/assembly looks like but I know there has to be a shaft which connects to the oil pump shaft (is there some type intermediate shaft?). There exists a retainer on GM OEM oil pump shafts (noted in image below) & I know a Melling oil pump accepts the hexagonal shaft. Your oil pump installed w/ an original oil pump shaft (hexagonal)? The pressed-on retainer (image below) of the oil pump shaft does nothing after the distributor is mated inside the oil pump shaft. If it becomes dislodged it would/should fall to its final resting place at the bottom of the oil pan. However/Unless/Possibly that retainer came off, is loose or whatever & is stuck on the shaft in some precarious position creating the noise? Again, it sounds ~1/2 engine speed. You mentioned distributor/gear in an earlier post and again in another post stating the noise sounds familiar w/ the distributor cap removed manually turning distributor. That retainer clicking bouncing about? You drop something (nut, washer, wedding band, etc.) down the distributor shaft causeway?

 

I counted 50 "ticks" several times with a stop watch and came up with about 175-180 per minute which would actually be 1/4 engine speed IF his 700 rpm idle speed is correct.
Strange noise for sure and I would still look at an exhaust leak.

 

It sounds like it's coming from the area of the starter which op have already mentioned. I noticed the starter is an after market one and possible it might need a shim? The noise appears after the car warms up because the metal expands and what ever is touching then starts making the sound. Once you're under way you probably don't hear it because of all the other car noises.

 

Bill - Good for you on the counts. I didn't perform a detailed count - good for you. I initially thought exhaust manifold leak, didn't mention it then you mentioned it & agree it sounds plausible. What's peculiar is the notion w/ distributor cap removed turning distributor by hand sorta/kinda sounds similar to noise at warm idle? Mysterious...

 

At work we used to deal with strange mechanical vibration frequencies related to ball bearings in our scan motors. We had tables with calculated ball retainer, ball rotation, ball-contacting inner race and ball contacting outer race frequencies based on the shaft rotation speed. The approx. 1/4 crankshaft frequency of this marbling noise would be close to a ball-bearing retainer rotation rate on something related to the camshaft speed. However, I'm not aware of any ball bearings in an Olds engine (not sure about the oil pump and distributor bearings, I always assumed they were brass bushings).

Another thought: The "inter-modulation" of the amplitude of the marbling noise may be the result of "beat frequencies" between something rotating and something vibrating.

Final thought: Could it be a bad spot on the timing chain and one of the sprockets that only makes the marbling noise when the two bad spots line up together? I'm pretty sure the number of links in the chain is not a factor of 2.0000 of the number of teeth on the sprockets. This could result in the period of the noise being longer than a camshaft rotation.

 

The MSD ready to run distributor gear style is the same as any Olds distributor (execpt the inner diameter for the shaft itself being 0.500), with the hex for the oil pump drive shaft. When i had the distibutor out of the engine, i checked if the pressed-on retainer of the drive shaft is still attached. When it would be off, it would be possible to pull the drive shaft out through the distributor hole and its not (for sure with enough force to pull the shaft through the retainer). So it seems that the retainer is still in its place.

No, nothing fell down the distributor hole. This noise is here since i have the car, and a few years ago, when i had the engine out of the car, i replaced the oil pan seal, so even if some pre owner let something fall in the crank case, i would have found it when i had the oil pan off.

 

I will check for exhaust leaks again, and i have a very small leak of the driver sides header, but nearly not hearable. Anyhow, shouldnt a exhaust leak maje more a hissing/ticking sound than marbeling?

 

Yes, its best hearable under car in the rear engine area.

Starter is a aftermarket mini-starter, and it seems like there is plenty of room between starter pinion/sprocket and flexplate when retracted. I once saw a sparter sprocket hitting a flexplate when engine running and ot made a terrible high-pitched metallic screeching noise, not a dull marbling like my noise, so i guess this is not the source.

 

my timing set is a double roller cloyes set, which is a very nice quality part. I highly doubt that this is the source. Even, the engine made that noise with its earlier comp-timing set too.

 

Thanks a lot for all the input, help and thoughts about this issue!

I will try further to find the source of the noise, just hoping that its not something bearing/rod/wrist pin/piston slap related.

I would like to drop the pan to insect the rotating assembly, but since im sure its a pain with the lifting up-engine in car-method, maybe i will pull the engine in winter to take a closer look.

The thing what calms me a bit is that it seems like the source of this noise is no really serious problem, otherwise, the engine wouldnt run through all those years without an issue except this noise.

One more thing comes to my mind is the fuel pump (lever on excentric), but when i "stethoscoped" it, i hear nothing.

 

Do you have a helper ? Bolt the converter back up to the flexplate and then have someone "power brake" the car while you are under it listening. Usually power braking it will make an exhaust leak louder.

 

This is a very wild shot, but a guy I once worked with said his family car had a soft knock to it the entire time they owned it. Unknown number of years. Anyway, they finally pulled the engine and took it apart and found the crankshaft had split in the middle of a rod journal. The connecting rod was holding the two halves of the crank together. This was a low performance inline 6 cylinder in an old Rambler or somesuch so not a lot of pressure on the engine. Highly unlikely, but a possibility.

 

Just a thought, pull the distributor and see if the bottom of the gear is showing wear.

 

Hard to believe that thats possible that a engine would not fall apart at some RPM with a 2-piece crankshaft, but i can imagine that it could happen when it mostly run at low rpms.

thanks for the help! I had the distributor out. No wear on the bottom of the drive gear, but i figured out that when moving the oil pump drive shaft sideways around with my hand, and its touching the block (the hole of the engine block where it goes through) the noise is quite similar to the noise it makes at hot idle.

I pulled the engine today, because bad autumn weather and winters coming soon anyways.

I will put it on my engine stand and remove the oil pan to take a closer look on the rotating assembly and oil pump + drive shaft.