S-2000 grunbling noise

Maybe you guys can help me out, im getting a sound from my 2000 S2000 while taking off from a stop under normal driving. I can duplicate the sound if i brake and release the clutch slowly to drop the idle down and get the eng/trans to shake, it almost sounds like some type of exaust rattle. But i have checked everything and this noise is driving me crazy....any input? Im guessing its in the trans? possibly the starter drive ? Igve had this car on the lift several times and just cant seem to pinpoint it.