Wheel bearing or rear diff?

[JulianH]

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?

[forester2002s]

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?

[JulianH]

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.

[JulianH]

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.

[Fairtax4me]

Have you checked the fluid?

 

How long has it been making noise?

[JulianH]

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.

[JulianH]

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.