That might be a rattle of some kind but it's not clutch chatter. Clutch chatter happens when you're accelerating and the clutch disc slips and rattles back and forth between the flywheel and pressure plate.
What you might have is a bad throwout bearing or a rattling clutch fork. Or it might just be the idle dropping down when the a/c engages that's causing the engine to lug a little bit and make something completely unrelated rattle.
If it gets worse, take it in and have somebody verify the problem for you.