check for loose motor mounts first, as that's easy.
You can also check the pressure plate mounting bolts to see if they're tight or not through the clutch inspection hole.
I have noticed most "rebuilt" pressure plates lately are not well adjusted. You can check the levers through the clutch inspection plate. I check the levers by placing the throwout bearing face against them lightly and checking them with a feeler gauge to see if they're all aligned evenly. I have seen them so far off that you can tell with the naked eye. They can be adjusted in place by breaking the stake on the ones that need adjusting and re- adjusting them. Once they're all even. re-stake the nuts. I like to apply a little bluing to the throwout bearing and lightly contact the levers, rotate the flywheel, then check the lever contacts for blue, indicating even alignment.
Another cause of chatter is oil on the flywheel or disk. If your car is not leaking, that's probably unlikely. But I have sprayed brake cleaner on the clutch before to clean it off with success on a moderately dirty clutch.
One more thing to check is that the throwout bearing is evenly pressed onto the hub. If its not, it will engage the clutch at an angle, causing chatter. The cure for that is to remove it and press it on straight...yucck.
there's a good notebook article on Ahooga regarding pressure plates. It describes the work on a bench, but reading it will give you an idea what to look for in determining if that's the cause:
see:
http://www.ahooga.com/notebook/howto/1/howto1.shtml
