Strange ground loop hum

[inscrewtable]

I say strange because at first I thought it wasn't strange, I have a couple of krkvxt6's plugged into my rMBP via the microphone jack. There was hum in one of the speakers so there's no surprise there. After examining the wire I see that there is some damage, so I ordered a new cheapo cable which is mini jack to two XLR's, seem pretty crap chinese quality but I felt that they would work fine. Then added a right angle minijack adapter which puts less stress on the mini jack because I have my MB elevated.

Anyway so now the strange part. After plugging them in to the new set up. There was no hum. Great.

After a while there was hum in one speaker, and I decided to think about it later. Anyway I send an email, and then there is the big 'whoosh' as the email is sent and then silence, no hum. Then after a little while the hum kicks in. I have repeated this a few times and definitely the hum stops after I send an email, then starts up again a short while later. That is what is strange. The hum appears to be coming from the macbook itself, and somehow when it generates the mail whoosh sound that somehow is doing something that supresses the hum for a while. Does this make any sense to anybody? Because this just seems weird. Could this be a problem with the mac, because it's already had a new logic board installed under warranty.

If I can get my hands on a a decent priced thunderbolt dock then I can plug a soundcard into that and plug the speakers into the soundcard, but I'm not sure if that is going to help.


EDIT: I have just discovered it's not the mail app per se it's anything that generates sound, if I blip the sound for a second on iTunes then like the mail whoosh the hum is silent then it kicks back in again after about 30 seconds.

 

[inscrewtable]

OK, I've been faffing around all morning and it appears to be a problem with the mac so I'm taking it in. Even if I play a completely silent track with no signal on it, like Roland Kirk's 'telephone conversation' for example, when I pause the silence in iTunes there is no speaker hum, then there is a click and the hum starts at exactly 32.5 seconds, if I blip the pause/play button the hum stops for 32.5 seconds then kicks back in.

The 32.5 second click can be heard through the headphones in the headphone jack but there's no hum through them.

curiously if I plug in a behringer u-control into the usb port and then set the system sound to the u-control then there is no 32.5 second click or hum, other than some very low level hum that is to be expected.

So it is definitely a computer internal switching thing and nothing at all to do with wires or other interference. It also makes a difference if the sound is set to system or some other output.