I've never heard anything like this from an engine. My best guess is something fell off into the oil pan but could it be some sort of bad knocking? Has anyone encountered this before?
Thanks

 

I recognize your previous posts and several of the issues you experienced.
Is the car capable of starting without starter fluid?
Has the car been able to run longer than six minutes?
Without an audio file it's nearly impossible for anyone to understand what sound you are referring to. Record the engine sound you're experiencing and post it either here (if short enough) or on YouTube and provide the link here.
If something "fell" into the oil pan, it would not be making any noise - it would be laying at the bottom of the oil pan and that is where it would remain - lifeless & still. There's nothing there to bang on it. A simple oil pump with pick-up tube and attached pick-up tube screen.

 

Are you familiar with the sound of preignition?

 

A new coil solved the starting issue and the brake in went great after fixing the shorted tach wire. I can try to get a video tomorrow; it sounds just like a loose part rattling but the brackets and exhaust are on tight so it's not either of those. That's a good point about nothing moving in the oil pan, so it's probably not anything in there.

 

Good deal. Hopefully you'll get a video of the noise posted; until then:
(1) Did the noise begin during the engine rebuild break-in period or sometime later after break-in?
(2) Did the noise gradually begin to occur during the break-in period or did the noise suddenly appear?
(3) Is the noise synchronized with engine RPM, i.e. slower noise cadence at idle and faster noise cadence with faster RPM?
(4) Did you pull the engine for rebuild or perform the rebuild w/ engine in car?
(5) Did the rebuild involve only the engine top-end?
(6) Does the noise sound like it's coming from the lower-end of engine or top of engine e.g. you've previously mentioned oil pan &/or exhaust area (lower-end)?
(7) Does the noise sound "tinny" (metal coffee can of tiny nuts & bolts rattling) or similar to the sound of a muffler rattling, exhaust clamps rattling, exhaust pipes rattling against the bumper (not saying that has anything to do w/ the sound - just trying to identify the type/pitch of sound)?
(8) Is the noise similar to a "rasping" noise e.g. sandpaper?
(9) Or, as you mentioned the sound is clearly a "knock" - not tinny, not high-pitched, instead a deeper (not rattling) lower pitch?
(10) Can you hear the noise at road speed or only while the car is stationary?

 

Alright here's the sound I'm getting. It just started yesterday, before that it sounded great.

 

Check your torque converter bolts to start

 

We can hear the sound. Please address the remaining questions in Post #5

 

Based upon sound alone, I tend to agree w/ 70W-32 - it sounds bottom end and torque converter-to-flexplate bolts. Did you pull the engine for the rebuild (that's why we ask pertinent questions - see Post #5 - again)? Did you disconnect the transmission from the engine? Please, begin to ANSWER questions if you desire further help.

 

The whole engine assembly was removed and entire engine was rebuilt. (camshaft and lifters, crankshaft, pistons, and oil pump were replaced). The noise appeared suddenly when I started the car yesterday; it was not present during break in. It's difficult to determine exactly where the noise is coming from, but it seems to be the bottom or the back; it could easily be coming the flexplate.

 

OK. That helps significantly in addressing where to look further. As already suggested, check out the flexplate torque converter bolts. There should be a cover over the converter where it meets the flexplate. Remove the cover and see if you see any movement and/or loose bolts.

 

What year is your car? You have the CSM?

 

Well, I got underneath the car and those torque converter bolts had gotten so loose, they turned by hand. I tightened them down and that was it. Thank you for the help.

 

Sweet. That saved you some time and a headache. Glad you found the issue & thank you for following up!

 

It’s nice you found the problem but if it ran with loose torque converter bolts the bolt holes in the flexplate may be egg shaped and the bolts may look like a beaver was chewing at them. Simply tightening them is an incomplete repair.
It would be prudent to remove each torque converter bolt one at a time and carefully inspect the flexplate and the bolts.

 

Wow. Great suggestion Dyno!