I know, I drive an e36, but I am having this problem too. Try to isolate the noise - you say it's from the rear, make sure it isn't heat shields back there, or break calipers (more of a tinny rattle than a creak). That's probably not it though - check the shocks by pushing down on the bumper and making sure your car pops back up once without bouncing. Or creaking. If it's not that, (I'm afraid this is what's wrong with my car) it could be sub-frame failure. I think (I'm not sure) this just happens from crappy american roads. For more information, search the e36 forum for subframe failure.
1998 BMW e36 M3 Sedan
Crap, I think I may be a yuppie...
2003 MINI Cooper