Hi All. Happy with my new iMac 24" 2.8, except for an audio problem thats bugging me. When I use my Edirol UA-20 USB audio interface I get a constant buzzing in the audio out. I don't have the problem when I use the headphone audio output from the iMac. Any ideas on how to solve this (apart from buying another audio interface), as I really need the audio in/out of the interface. Thanks.