Weird audio phase cancellation when capturing with iSight

[stringtheorist]

When I first got my iMac (OSX 10.5.2 with Leopard) I had no problem capturing audio/video in iMovie using my built-in iSight. Lately however I get a strange, undulating phase-cancellation sound appearing on the audio track about 10 seconds after I start recording. I can't hear this through my Mac when I'm recording, only during playback. It completely messes up my guitar movies.

I have since downloaded iMovie '06 but I get the same problem there too. Is there a fault with my internal mic? What else could be wrong?

Thanks in advance.

[Poeben]

You're not monitoring your audio through speakers while recording are you?

[stringtheorist]

Yes but I've always done that and never had any problems before. Besides, I tried it with the speakers turned off and I still get it.