rear wheel rattling

[isotopeman]

I've had a rattling noise coming from the rear passenger-side wheel for awhile. I could move the wheel and hub by hand with it on the ground, so I assumed the bearings were bad. I could hear that whirring/humming sound as well while driving.
A nearby shop replaced the hub and wheel bearings with a new set for 160$. The whirring noise is gone, but the rattle is still there. It's usually when I hit a bump or something. I've tried to shake the spring and strut with all 4 wheels on the ground, but could not repeat the noise. My brother and I shook the hell out of the car with it on the ground but could not repeat the noise. We shook from the wheel, and although the wheel and hub seemed secure, the noise was there.
Does anybody have any ideas of what to try to replace next? It's more than a month before I get paid again, so I can't just keep buying parts that I think might be culprit.
The near-constant rattling (we don't have perfect streets anywhere do we?) is driving me nuts!

[Legacy777]

Have you tried jacking up the car and getting underneath it to see if anything looks out of whack?

[n2x4]

Check the trailing arm and lateral link bushings, and like Josh said, take a look for anything phunky.

[evolutionmovement]

Yeah, I'd check for worn bushings. The other thing it might be is a broken strut rod at the top mount. Take a socket to the nut and see if it turns. If it turns easy, it's broken. On a wagon, you can use a crescent wrench on it and see if the rod end rotates with the nut, but I'm not sure if you can see it in the trunk of the sedan. The car may seem to otherwise ride and handle fine except for the noise, but the broken off end underneath will be beating the hell out of your top mount, likely necessitating replacement of it as well. A frozen top mount may actually be what causes the strut to break up there in the first place for all I know, but all the OEM struts I replaced on my car went in this manner.

[isotopeman]

Well, I checked all these things with the wheel on as it's easier to shake that way. I could still hear the rattling, but it wasn't the bushings, mounts, or any of those things. I wouldn't have guessed what it was.
It was the rotor. I found it when I finally took the wheel off and shook what I could. The rotor rotates around the same axis as the wheel with the slack between the holes and the studs that go through them. It also moves in and out as far as the pads will let it. I noticed that the part of the caliper that holds the piston moves as well, but not the pads and outer part of the caliper. I'm replacing the pads and have them already, but shouldn't the mounted wheel keep the rotor from moving? I don't have Subaru OEM wheels. Maybe they don't fit perfectly around the hub?

[IronMonkeyL255]

Probably they don't fit right.

With wheels with a proper center bore size, the back of the wheel sits against the rotor, holding it tight against the hub assembly.

The caliper is meant to slide around like that, so that isn't anything odd.

[n2x4]

If the wheel's center bore is too small, you're just asking for trouble. That was the case on my last car. You're putting extra stress on the wheel studs and just asking for some to break off.

I did this on a car once. I didn't know the bore was off when I bought aftermarket wheels and I broke off 3 studs when I hit the brakes. Luckily there were still 2 ok, but the wheel wobbled like crazy as I pulled off the road.

I just ground out the center enough so it wasn't loose and went up against the rotor flush.

[isotopeman]

Thanks for the info guys; it's all been useful.
I rotated the wheels, and that took care of the wheel fit problem. (They're steel 15" wheels from a friend's VW. My car didn't have two alike wheels when I bought it.)
The rattling was better, but there was still something going on. I checked the top mount as you suggested, evolutionmovement, and though the strut seemed secure when I tried it from the wheel well, the 17mm nut and 5mm allen stud both turn quite easily (together).
I guess my question should have been "What's not rattling back there?".
Thanks again.