Perhaps the Motor Mounts . When Clutch is depressed it puts pressure on the whole motor from the pressure Plate and can move the motor around however with a " hydraulic " operated Clutch this does not make much sense humm
Here is a post that addresses the same sort of issues
Clutch pedal pulsating/ car shaking - BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)