how did you fill your tranny fluid? did you pump it from the drain hole, or did you actually fill it from the fill hole on the side of the case and allow any excess to drain out from the fill hole before plugging it up?

if i understand you correctly this always happens as you are engaging and/or disengaging the clutch.

i had a similar problem once and it was b/c the quick lube place re-filled my trans from the drain hole and overfilled it. the excess fluid seeped out the output shaft bearing and soaked my clutch and pp causing it to chatter violently when engaging/disengaging. a motor mount broke at the same time either by coincidence or b/c the motor kept rocking so badly from the clutch issue.

my vote is bad clutch/pp possibly from too much fluid and/or broken motor mount.