Audio problems with FaceTime

I've been unable to use Facetime on my MacBook AIr (bought it spring 2012, running OS 10.8.2) I don't have any problems using it on my iMac. What happens on the air is I get a very loud noise as soon as I connect a call, and it doesn't go away until I disconnect. Not sure what the best way is to explain the noise, but it is very loud (can't hear the person on the call over it) and it sounds like the computer is dying or something. Any ideas what is wrong or how to fix this?? Thanks!