houseofhendrix

I bought an 1987 4000S a couple of days ago and there is a noise coming from the rear end that I am not comfortable with. The closest description I can give is a loud hum, kind of like you would get with off road tires on a large truck. It is dependent on speed and happens regardless of the car being in gear, in neutral, or if I have the clutch depressed or not. The tires look nearly new. The dealer said he thought it was road noise from the tires, but I do not know Audi's well enough to know if this is the case or not. There are no odd smells or handling issues when driving the car. What do you think? Could it just be tire noise or something more serious? Thanks for any advice.

Aaron

Rogviler

If it's only when driving then it's almost certainly the differential. Most of them that I've heard hum or whine. About all you can do is drain and fill the fluid or at least top it off.

Also possible that it's a CV joint starting to go. Typically their noise is more dependent on load (acceleration rather than coasting or slowing) and cornering. Check for rips in the boots. Once they start to make vibration and noise there's not much you can do but you may be able to limp it along for awhile by cleaning the joint, repacking with grease, and installing a new boot.

If it happens even when parked but only with the engine running then it's the fuel pump. They all make different degrees of noise, but normally it's a buzz and only gets really loud when you're almost out of gas, have a clogged filter, or if it's starting to go out (but only if it was previously quiet, as some are loud to begin with).

-Rog

tenspeed

First thing I would do is to swap the tires. Criss cross them to see if the noise moves. While the rear is in the air, grab the spring and spin the tire by hand. If you feel a rough vibration, the rear wheel bearing is bad.

The dealer should have used a mechanics stethoscope on the rotating gear to localize the sounds. If there was no sound, it would be the tires. It's all part of determining the root cause of the problem.

houseofhendrix

[QUOTE=Rogviler;24725852]If it's only when driving then it's almost certainly the differential. Most of them that I've heard hum or whine. About all you can do is drain and fill the fluid or at least top it off.

This may be a silly question, but rear differential noise from a front wheel drive? Is that a thing?

Thanks again

Aaron

houseofhendrix

I will do the tire swap and see what happens, thanks for the advice!

Rogviler

I was assuming quattro. The FWD cars seem so rare...

In that case your only options are the tires or wheel bearings (since no CV joints or differential), as above. At least rear bearings are easy to change on a FWD.

-Rog

houseofhendrix

I'm guessing it's the bearings then. The tires don't seem aggressive enough to have that kind of road noise. The bearings are not particularly expensive so either way I'm not out a ton of cash. Thanks for the advice.

Aaron

Joeltron

I would also guess it's the bearings. I did mine a couple of summers ago. Mine had a hum just like bad tires, but there was no play in the wheels and no difference when cornering which is something everyone says to look for with bad bearings. None the less, I swapped 'em out and the noise is gone.

Joeltron

If you're thinking of doing it yourself, here's how I did it:
https://www.audiworld.com/forums/aud...l-car-2853106/

I assume it would be pretty much the same for FWD

Rogviler

With FWD, it's even easier than that, at least for the rears. Once you take the nut off, everything just comes apart. The only annoying part is putting it back together and getting the nut just tight enough but not too tight...

-Rog