Question: Rear driveline noise when decelerating

[Co-Vette]

I have a 93 convertible, all stock, auto with performance rear axle. Have been trying to get handle on a random noise coming from what sounds like the right rear. I can only hear it inside the car when the top and windows are up. Have been hearing it off and on for about a year, it's not getting any worse but it's not right either.

If I take my foot off the gas between 50 and 30, I may or may not get a "thuppa thuppa thuppa" noise that goes with the rotation of the wheels. Varies in intensity. Does not change when I apply the brakes. Once I'm at 20 mph or less it stops.

No noises when I accelerate slowly or quickly.

I've had problems with the control rods coming loose before, but that creates a noise most noticeable at low speeds when braking. Checked those, all are tight and torqued to spec.

I had a local ASE tech check the driveshaft, half shafts, bearings and u joints. He drove it for a while but could not hear the noise. I suspect he drove it cold and the noise seems more prominent after the car's been driven a while.

I did a search but most threads are dealing with noises on acceleration and the solutions are usually bearings and u joints. Smack me if I missed a thread that applies to my issue, but I thought I would post to see if anyone had any ideas I could check before covering the shop manager's next boat payment...

 

[Schrade]

Try dropping the tranny into neutral as soon as you let off of the gas, and see if the sounds are the same.

This COULD help distinguish a driveshafts/halfshafts problem, from a tranny/torque convertor problem...

 

[boomdriver]

Yep... Shrades right.

When you hear the noise, shift to neutral and see if that changes things.

One thing that I KNOW to be a possible issue is owners and service people failing to add the GM limited slip additive to their rear end fluid. That alone causes odd noises, some reduced performance as far as traction in either/or wheel and internal wear.

The rear end is about as bullet proof as any part of the Corvette, IF the fluid is maintained and that means adding that tube of stuff to fresh lube.

Sorry, I cannot tell you how much. I am positive that someone here has that valuable information.

 

[Co-Vette]

Drove it today, shifted to neutral when I heard the noise, it continued whether in or out of gear. Also downshifted to 3d gear, same noise, no change. Also no change going around curves. The wife heard it too and confirmed I'm not just hearing things (in fact she says it's rear pasenger side). Along with any ideas here, there are a few wrenches in my club that said they'd take a ride with me later this week. Thanks for the advice so far.

 

[boomdriver]

Drove it today, shifted to neutral when I heard the noise, it continued whether in or out of gear. Also downshifted to 3d gear, same noise, no change. Also no change going around curves. The wife heard it too and confirmed I'm not just hearing things (in fact she says it's rear pasenger side). Along with any ideas here, there are a few wrenches in my club that said they'd take a ride with me later this week. Thanks for the advice so far.

That tells us that the sound is in the drive line or wheels.

I'd check out the wheel bearings myself. Too many shop guys are not experienced with Corvette specs on bearings and will let failed hubs pass as good 'nuff.

Jack the car to get rear wheel off the ground. Grab wheel at 12:00 and 6:00 position and try to wiggle in/out. if you can feel any movement the bearing is suspect. if you can SEE any movement, the bearing is junk.
The hub/wheel bearings should be so tight that they do not wiggle. A very slight amount of play is accpetable but any more than that indicates a worn out bearing. These are throw away hubs with the bearings made into the hub. Good ones are'nt cheap but will last. Cheap knock-off's can be had under $150 but many come as loose as the old one, just less noise. They usually don't last very long either. 80,000 miles (or more) is a good number for a hub bearing thats been kept clean and installed correctly. Also check the 3 hub mounting bolts for tightness. They DO work loose sometimes. I've had it happen. They are torx and can be seen by looking along the half-shaft toward the spindle/hub.

While its in the air, grasp at 3 & 9:00 also.....this test the tie-rod ends/ rear toe rod assembly. There should be NO movement there.

next, try rolling the wheel back/forth by hand while its in the air to see if you can duplicate the noise.

Last at home test....
back on the ground,
apply power while holding the brake and closely observe the rear wheel to see if it twist around as torque is applied and released. The purpose of that rear suspension 5 pt link system is to prevent that while allowing the power to turn the wheel. Get someone to assist with the passenger side...

 

[Co-Vette]

Thanks much...will run these checks later this week and post an update.

 

[Co-Vette]

Raised the rear of the car last night, pulled on the wheel at 12-6 and 9-3 as suggested, no play either way. Put it in neutral and spun the right rear wheel by hand. No noise from the wheel, but I did notice a quiet but consistent click, 3 times for each turn of the wheel, sounds like it's coming from the rear u joint (going into the diff). I don't hear it while I'm driving though, and my prior experience with bad u joints is a chirp noise when you accelerate.

Since I don't have the time, lift or tools to do it right I took it back to the shop for them to check and swap out if that joint is bad even if it's not the cause of the original problem. If I get a fix on it I will update.

 

[boomdriver]

Raised the rear of the car last night, pulled on the wheel at 12-6 and 9-3 as suggested, no play either way. Put it in neutral and spun the right rear wheel by hand. No noise from the wheel, but I did notice a quiet but consistent click, 3 times for each turn of the wheel, sounds like it's coming from the rear u joint (going into the diff). I don't hear it while I'm driving though, and my prior experience with bad u joints is a chirp noise when you accelerate.

Since I don't have the time, lift or tools to do it right I took it back to the shop for them to check and swap out if that joint is bad even if it's not the cause of the original problem. If I get a fix on it I will update.

They can easily make noise before you can feel them loose.
Once pulled apart, I am positive that you will find a dry joint cap with loose needles inside. 3 clicks to 1 rotation spells drive shaft joint, anyway you look at it.

I learned how to do this at home in the garage...as long as you have faith in your jack stands its not difficult. To do half shafts I'll disconnect everything and lay under the rear and push the wheel away with my feet...installation the same way. beats hell out of trying to hold things with one hand and start bolts/nuts with the other or work around the spindle and rods. When doing it this way, its just a matter of removing joint straps..

Drive shaft is a bit more challenging, but I prefer to do it myself so I KNOW the C-Beam is bolted and torqed properly.

 

[Co-Vette]

Update

Took the car to the shop. Once on the lift, isolated the clicking noise in the area near the vehicle speed sensor gears. Since that requires some teardown I'm going to hold off on that fix unless it gets worse or if I have codes or speedo problems.

On the thumping noise - I took the shop's master tech for a ride, after about 20 minutes he heard the noise as well, but from inside the car and the type of noise, like me he couldn't isolate it. Inspected all joints and bearings and the read end with electronic stehoscope. Couldn't reproduce it on the lift or on the dyno. Runout on rear rotors is less that 2/1000, spec is 4/1000. Calipers, suspension and and ebrake cables are tight and operating as they should. About 3 hours of labor/diagnosis but they sent me on my way with no bill, I guess because I use them for servicing the rest of my "fleet" of 20th century cars.

Drove it home, slowed for the first traffic light and at about 20 miles per hour, thupp-thupp-thupp until I slowed to about 5 miles per hour. Didn't do it again the rest of the way home.

Solution, keep driving it to identify more symptoms and get my gearhead buddies to go for a ride to see if they have ideas.

Thanks all for the suggestions.