Check your motor mounts for looseness. Just a little bounce, especially at the bellhousing, will cause that kind of symptom. The clutch linkage gets to moving back and forth as the bellhousing bounces, and it becomes a harmonic vibration that feeds on itself.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!