weird sound from the read while using A/C

hey guys, I've been hearing a really weird sound lately while using A/C and I have no idea what it could be. I have a new '05 ZX3 and was recently rear-ended (<$1000 of damages). However, now whenever I have the A/C on and I'm driving (doesn't do it while stopped), I hear a *tsk* type sound coming from the rear aprox every 7 seconds. Anyone have any idea what it could possibly be?