tire/ rim ratteling

a few weeks ago my front tires started to rattel after warming up (seems to rattle when the tire rotates, faster i drive, faster it rattles) so i went to my mechanic he said brakes were good and couldnt find any thing else, so i had him rotate the tires. now the same noise is coming from the back (which are now what were the front tires) and it is still making that noise. any body have any idea what it could be?

Here is a shot in the dark. I once had a rattle on my 1995 S10 Blazer. It also came from the wheels, which were factory aluminum alloys. They had ten holes (I think) and a plastic chrome center cap. Turns out the center caps were the culpret. The spring retainers on the back side of the center cap got loose/fatigued over the years. They were loose, but not loose enough to fall off. In my case, I put duct on the inner lip that contacts the wheel. That seemed to fill the gap. Kinda Mickey Mouse, but it worked.