Gang: I've started hearing a faint whirring sound when pressing the accelerator at times, mostly during cruising at higher speeds (50+), but never while coasting. No performance issues what so ever. Seems like it's coming from the drivetrain. Many think it's something with the differential. Assuming it is, what are the risks of driving it as-is for a while? Sometimes I'll take the car to work which is 40ish min of high speed/highway driving. I'll drive maybe 50-150 miles per month on average. 70 CS Conv 350 2bbl 122k. Thanks in advance for your serious and solid advice!

 

Personally, I would pop the cover, flush it out & inspect. If there is evidence of coming Armageddon, deal with it as appropriate. If not, refill with fresh oil and if appropriate, LSD additive and see if anything changes.

Might be a good idea to check the axle bearings, they are more likely to fail than anything in the diff and can be done independent of the differential.


​​​​​​….

 

Thank you!

 

Good suggestion to inspect and change the dif and ALL the other fluids on a 54 year old car...PS, brake, coolant, trans (do trans at least 2 to 3 times within 30-50 miles to get a good flush or get it power flushed) etc.

A good rule of troubleshooting is, well, troubleshooting. Dont go directly for the hardest problem.
How about starting out simple working your way to the larger more complex problems.

Tire balance? How old are the tires?

Try to identify where the noise/vibe is coming from...front or rear?
Do you feel anything in the steering wheel or in the drivers seat?

What happens if you briefly shift into neutral at the speeds you are observing the noise? Does it go away?

Thoroughly inspect the U-Joints by grabbing the shaft then pull up down side to side. Do you see, hear, feel, play?
Pull the shaft and articulate the joints to feel for bind or slop. See any rust dust emanating from the caps.

Can you get up/down movement on the pinion? If yes its time for inspection.

Whens the last time the front wheel bearings were repacked?
Rear brakes been inspected and adjusted? Dif grease leaking on the shoes can cause some weird noises & vibes as the shoes hang up on the drums as they heat up.

See where I'm going.

 

x3 on removing the differential cover. You also MAW replace those rear axle bearings.

 

Great advice and much appreciated! Had the car thoroughly inspected and fluids checked/topped off in April, if nothing else for safety's sake. All was good then and I've only put about 300 miles on it since. The car runs and drives as beautifully as when I got it nine years ago, except for a few new rattles with the windows 😄. My main concern is how much more I can drive w/o doing any real damage, and considering the noise is so quiet that it wouldn't be heard with the radio on or the windows down. Budget and time are not on my side at the moment, and if this can wait until Spring while I still occasionally drive it through the winter, that would be great for me. I have zero experience with a noise like this...
Ok thanks again

 

Topping off fluids is nice but with unknown service history, you could wind up with expensive damage or a safety issue that could have been prevented with little money upfront.
For example, the trans fluid could be 30-40+ years old. That means the oil's protection lubricant and corrosion inhibitor package has expired. The same goes for the coolant. Hook a volt/ohm meter up and probe the coolant. You should see .04 VDC. Any more the coolant is eating your radiator and freeze plugs.

We don't realize how fast time flies by with our hobby cars.

It's hard to say what is on its way out on your ride. I can't tell you if it will cause more damage because I don't know what it is. Generally, noise is the tip of the proverbial iceberg if ignored.

 

I had a pickup that was doing the same thing. I wound up changing the ring and pinion and setting the gear pattern correctly. It solved the problem. Just saying.

 

If the OP just started hearing the noise, its unlikely to be the gear pattern changing from good to bad (assuming pinion preload is still present). A certain amount of gear noise from the rear is not completely unusual. Every RWD / solid axle car I've owned or rode in generated some gear noise. Proximity to the rear gear and other noise sources makes a big difference in how much gear noise you hear. As a kid riding in the back of dad's '66 Impala wagon, you could hear the rear gear noise pretty clearly, but from the front seat, not at all. Gear noise does tend to be at a higher frequency than bearing noise (more "whining" than "humming"). Also, gear loading (accel / decel) changes the amplitude of the gear noise, not so much for bearing noise. While the fact that the noise increases while accelerating points to gear loading changes, it may be possible that the pinion bearings are going bad (if pinion preload is lost, then the gear pattern could change under changing load conditions. Loading on the outer wheel bearings changes insignificantly under subtle accel / decel load changes, but loading on the diff carrier bearings would change much more. For sure, its worth a cover removal / inspection / fluid change to get some peace of mind, less than a 2 hr job. You never know what you might find inside.