Okay ive had this problem for over a month (s4 10th ae turbo II)and have come to the conclusion that it not the front differential as i had suspected...ive replace the front diff mount on 4 different rx7's all being NA, and the thing is the clunking only happens under boost and high rpm when the most power engine power is produced...its pretty much stock boost...
i jacked the car up and realized that there isnt anything out of the ordinary but i would like to add that the lsd seems to be weak...the sound is like a heavy clunking that when in motion it slows me down really bad until i push the clutch....and it does it every gear if i decide to actually push the car.... normal driving however is fine....

 

it all happened after i serviced the diff... i put redline 75w90 with a bottle of lsd additive...

 

what happens when the two rear differential mounts go out? any particular noises?

 

to be more specific on the clunking its like a series of harsh hits that happen when driving hard full acceleration and as soon as the turbo spools for maximum boost.... it happens....