I am having audio playback distortion on my 2010 Mac Pro. When I am listening to (mostly) music there is a very subtle but noticeable distortion in the playback. The distortion does not last even a second but it can happen at least once every five minutes. Playback is of audio (mp3, m4a) through iTunes. I have tried with VLC and the distortion is still present. I say mostly music as I do listen to podcasts, movies etc and if it is happening with these then I cannot hear it. To try and solve this I have tried the following... Installed original Apple RAM. Reset NVRAM and PRAM. Re-installed ML (10.8.2) - with no other software installed. The Audio files have been on different hard drives - maybe it was a HDD on its way out, but I doubt all four of them are. Removed all internal cards, HDD and ran with only the boot drive. As above but with another drive. Plugged the Mac into a different mains socket. Unplugged my monitor - not sure why it just seemed a good thing to do. Unplugged all peripheral devices. Re-downloaded songs via iCloud - maybe I had a problem with the actual files. However as the files are from iTunes, Amazon, Play.com, CD rips etc I ruled this out. Plugged my headphones and speakers into the front audio jack (I usually have it plugged in the rear). As above but with another set of known working headphones. Needless to say none of the above worked. Then comes AppleCare - who didn't actually get me to do a lot. As I have tried all the above they only had me remove two plist files from Boot Drive/Library/Preferences/Audio but it didn't work. The files were not recreated on a reboot and with the files missing iTunes always froze. The Mac Pro was sent away for repair where it had its logic board replaced which didn't make one bit of difference. A few days later it was sent away for another repair where it had the PSU, GFX Card, processor tray, (I think CPU) replaced. It once again did not make one bit of difference - within fifteen or so minutes I could hear the audio playback problem. It was actually returned to me today. In total the Mac Pro has been away for repair for one whole month. Before I phone AppleCare yet again am I missing something obvious? Any ideas?