legacycentral bbs

The advise here is great so I have another question. I own a 1991 Subaru Legacy 4WD, non turbo with an automatic trans from a 1990 Legacy. That swap was done prior to me being the owner. I have no idea when. Now there is a grinding noise coming from somewhere when I back out of my garage. I'm thinking it might be the transmission or the rear end. Anyone out there with a similar issue? Or is there an issue with the reverse gear that I didn't know about.
Also the grinding noise only happens when I back out of my garage, no where else. even when I back out of a space at Wal-Mart. Something might be cold, all of the fluids check out.
Trans fluid is full and clean, no boot leaks or tears, no rear diff leaks, no front diff leaks, but teh front diff measures between full and low. I look up Google for info on the front diff, but nothing. Any info on the front diff, or if it's part of my problem?

Park on the street one night and back it up in the morning to see if the sound follows it.

Well there's news. I cleaned out the spaces between the rotors and the dust shields. No rocks found, no dust. But the grinding remains, but has become more of a squeal. also the sound sometimes happens elsewhere now at very low speeds and when I make right hand turns. Even not in reverse. So it's coming from the left rear; that's my guess. Maybe somewhere else in the rear. I still can't figure out what it is.
I'm going to have the front and rear differentals serviced this weekend, if I can't figure out what the heck is making the noise maybe the shop can.
But I'd like to figure it out first. Anyone have any ideas?

wheel bearing I bet.

Maybe but wheel bearings are usually a constant speed dependent whirring or thumping.

I still think its the brakes. Either a heatshield or a brake pad.

That's what I'm betting on. Left rear. I wonder why just that one, but I'd rather not know at the moment. How hard is it to replace a rear wheel bearing? Google brought up about $55 avg. for the part. Now do I replace one, or both rear bearings?
I'm taking it in for some checks and services this Saturday, so that'll be the day work will be done and completed.

Also it's been doing the grind on and off (only at very low speeds, and in low speed reverse) for about a 3/4 week now. Can it last until Saturday if that is the issue?
I'm just a little weary since I've replaced almost everything on my Subie. Even the engine with a rebuilt one.

I've personally driven on wheel bearings until the point that the wheel was almost falling off, the car couldnt track straight, it sounded like a bird filled lathe and it was smoking at stops.

Can you check the thickness of your brake pads?

The brakes were my initial thought. But all four discs check out. Pads are still good too. I thought axles as well, but no. And even the rear diff, but no to that as well. There has been a lot of construction on Rock Blvd (Sparks, NV) on the way to my g/f's house, but I doubt a rock would stay stuck in the brakes that long.

Pad thickness: Fronts are at about 85%, rears at about 30%.

I was going to put some new 07 Impreza WRX brakes, or a set of Wilwoods, but that's another discussion.