Thread External USB sound card problem on Mac Pro

Hello all !

So I recently acquired a 2010 Nehalem 2.8 Quad-Core Mac Pro with the intention to use it to make music (Mac OSX Snow Leopard 10.6.8 is currently installed). I've been using a 2008 black MacBook since, well, 2008, but I was starting to get tired of the constant fan noise when using CPU-intensive audio applications and the small screen so I wanted to have a computer that would better fit my need for silence.

The problem is, I was using an external USB sound card which I've had no problem with using it on the MacBook (it's an M-Audio MobilePre MK2 purchased around 2012 or 2013), but as soon as I hooked it up to the Mac Pro I noticed this really upsetting high-pitched buzzing sound.

Being familiar with this kind of problem I immediately thought it was a ground loop (the sound feels very electric) so I took the various steps detailed on the sound card company's website to get rid of ground loops (trying to hook up all the sockets to the same power outlet, trying different usb ports, swapping the usb cable for another usb cable, removing all other usb peripherals apart from the sound card, etc.), as well as updated to the latest drivers, to no avail.

But I noticed a really weird side-effect to the problem : the buzzing sound only comes on when audio is played. If the computer isn't sending any audio to the sound card, it's all perfectly silent, but as soon as I play a sound (regardless of the application), even if the volume is turned down completely, the buzzing instantly comes on, and stops as soon as the computer stops sending the sound or the sound itself ends.

I phoned up the audio hardware store where I first bought the sound card to get some more insight on the problem, as well as snooped around online for some answers (which turned up nothing). The guy at the store told me that, considering the fact that the noise only comes on when audio is played, it was unlikely to be a ground loop since, in the case of a ground loop, the buzzing sound is continuous and is heard regardless of audio data being sent or not (basically as soon as the speakers are on one should hear it) : so I was told it was probably a hardware compatibility issue between the sound card and the computer itself, to which the solution he proposed was to just get a new sound card.

Which I did, but second hand. I took great care to try and choose a different model from a different brand altogether, and found an Alesis i/O2 external USB sound card for not too high a price. But as soon as I hooked it up to the computer, I got the exact same problem as my other sound card. Tried again all of the steps to get rid of ground loops, just in case, to no avail.

So I'm kind of stuck here, it would seem like a hugely unlikely coincidence that two different sound cards from two different manufacturers would have the exact same problem (especially considering both soundcards work perfectly fine on the two other computers in the house), so it has to come from the computer, but I don't see what the problem could be, considering other usb peripherals I've tested out seem to work fine and the computer itself works perfectly fine as well and is in great condition.

I obviously haven't tried everything, I could still try to see if the problem is present when using different types of sound cards, either internal ones or ones that connect via Firewire, but I have limited cash to spend on buying new sound cards, especially since I just, you know, bought the computer.

Anyone encountered this problem before ? If you want I took a video of the problem that I can include if that helps.

Thanks in advance,
Alex.