Clutch or gearbox?

Hi all, issue with the gearbox or clutch. Car is a 2001 9-3 SE 2.0t.

Girlfriend notices it when at standstill, clutch pedal depressed and in first gear. Doesn't happen all the time but the car vibrates a bit (can't feel it in the pedal) and then calms down after about 5-10 seconds.
I don't experience it as I don't wait at traffic lights with the car is gear....

Any ideas as to what it is?

Julian