FaceTime audio cuts out, except when I hold down a keyboard key.

Hi, My 2011 macbook pro running Yosemite, keeps cutting out during FaceTime calls, they can still hear and see my perfectly, however I get a few loud crackles and then the audio cuts out, the strange thing is, as soon as I hold down any key on my keyboard the sound comes back, but as soon as I let go the sound goes again... I have tried hanging up and recalling, but the problem still persists I have a video if anyone would like to see it.

Cheers

Damian