Clattering 85

At a certain RPM in my car, there is a pronounced clattering that moves in time with the drive shaft. i know its not the timing chain because the noise only occurs when the car is moving and under acceleration or load. its not the drive shaft because the noise only occurs at a certain RPM, and if the whole drive shaft were at fault, it would happen all the time. if you can help me please do...

Lots of possibilities come to mind, the two that come to mind given your situation is possibly a seperated harmonic balancer or a tranny problem.
Good Luck.