Wheel noise -- bad bearings or something else?

Hard to tell from a recording, but the scraping sound while spinning the wheels sounds like normal brake noise to me. During actual driving, the pads are normally pushed back a little by vibrations, so they don't rub as much as when spinning by hand.

Unevenly worn tires can sound very much like bad wheel bearings. Even my experienced Toyota mechanic was not sure what it was, when I had noises. The only way to find out for sure before disassembling, was to swap the tires with new ones or used ones which are known to be good. In my case, it once turned out to be the tires, and another time it really was the bearings.

Also could be flat spots or bulges on tires, balancing, out of round or wobbly rims, a bad shock absorber ...

 

The mechanic might have screwed up the rear caliper piston orientation when reassembling. It's very easy to do. There's a cross-pattern on the face of the piston that MUST be oriented like a letter X, and BE WELL SEATED THUS:



(Repair Manual extract attached)

I would suggest you raise the rear and try spinning the wheels. There's normally "slight" drag, but they should easily spin 2~3 revolutions:

 

Yeah... I'd have the mechanic confirm their work based on Mendel's post... If the noise wasn't around before the mechanic worked on it I'd be reluctant to think it's the bearings until they get really bad and way noisier. And if you can get a noise from the wheels when car is jacked up I'd think that decreases the odds of it being a wheel bearing.

 

I'm calling it now. Slide pins. My bet is they need grease.

 

Any update?

 

Those slide/glide pins are the worst cultprit for friction created due to not floating at all. I even removed one that was bent and unable to move…

Lubricant here is a touchy subject, but I use only thin layer of silicon grease when cleaned and/or replacing.

 

I don't know that there's anything touchy about it, just that like anything else, readers are entitled to know what Toyota's recommendation is. Toyota (for whatever reason, I don't know, I don't work there) specifies a "lithium soap based glycol grease"; if you get it from them, it's called Rubber Grease and it's red. 08887-01206 is the part number if you buy a whole tube, but that's often unnecessary because whenever you buy their caliper rebuild rubber kit there's a little packet of the right stuff in it.

They do specify silicone greases (a couple different ones, even) for use on other parts of the brakes. They have diagrams with different-style arrows showing which parts of the brake get which kind of grease. (The diagrams have been posted here before.)

My own experience, somehow, has never borne out the slide pins being "the worst culprit". When I have worked on dragging Prius brakes, I've most often found sticking of the pad ears in the "fitting kit" clips, and even found a piston simply failing to return (not seized, easy to move, but simply missing the 0.3 mm or so of return it's supposed to have when the pressure is let off and the internal rubber seal returns to its resting state) more than once, and that's more times than I've found a slide pin to be the culprit.

Other than moving each slide pin a little bit in and out on each brake inspection, I generally don't even find one needing attention unless something has damaged its rubber boot and let salty water in.

I wonder sometimes if "slide pins" aren't a bit like "fuses"—the parts out of a whole coordinated system that happen to be the ones most people have heard of, so they get posted about that much more, which feeds back into the cycle. Sort of like in so many posts about the car doing something funny electrically, no matter what the something funny is, the first post is likely to ask where's the fuse to change to fix it.