I have a terrible hum/whine coming from the rear of my 91 Legacy(220K). I thought it was a wheel bearing, but when I pulled the wheels, calipers, and rotors there was no play or sound. I got in the back seat/trunk area to try to get a better idea of what it was, I couldn't definitively say it was from the left or right. When you take your foot off the gas, it does get an additional noise, what I would call a "gear" noise, which has me wondering about the diff. I recorded a short video...

 

 

How can I definitely rule out a wheel bearing or the diff without tearing into the hubs?

Bad wheel bearings can be hard to identify.

I had a noisy 'rumble' from the rear, gradually getting worse, but I couldn't figure out if it was left or right.

- I tried an infrared thermometer - no temperature difference between left and right.

- I tried jacking up the wheels and rocking the tires - couldn't feel anything. Tried that many times.

- I tried a stethoscope on the hubs while rocking the wheels - couldn't hear anything.

Then, eventually, when doing the rocking test, I heard a very faint 'click', couldn't feel it just heard it.

Had that wheel bearing changed, and all was quiet.

 

Having said that, your noise is more of a 'whine', which I would expect to hear from a noisy gear-set.

So it might be the diff.

Does the diff get warm after driving at high-speed?

Having said that, your noise is more of a 'whine', which I would expect to hear from a noisy gear-set.

So it might be the diff.

Does the diff get warm after driving at high-speed?

 

That's a good question... I'll go take it for a quick drive a mile down the highway and see.

OK, yes, the diff is a bit warm. 2 mile round trip on the highway.

 

Also, it feels like there is resistance when you start off - like driving through snow, or with your parking brake on. You hit about 30mph and it "lets go" - meaning the resistance/drag goes away until you stop the car again. The sound becomes audible around 20mph.

Have you checked the fluid?

 

How long has it been making noise?

Diff fluid was black, thick, and smelly, but no chunks of metal. There was a little bit of the normal "filings" on the magnetic plug, actually less than I've seen on some of the fluid change videos on YouTube. It might have also been a bit low - it's definitely leaking oil from somewhere, looks like maybe it's leaking past the axle shaft seals. I filled it up with fresh fluid and took it for a drive. No difference. I'll spray the diff off with engine cleaner to see if I can tell where it's leaking from.

 

I had my wife drive past me a few times so I could get a listen from the outside, I'd say it's coming from the driver's side. So I'm thinking it's the rear driver's side wheel bearing at this point.

Incidentally, it did wind up being the rear diff. Gears look fine, it must be a bad input bearing. Grabbed a lower mileage replacement down at the wrecking yard for $50.