Reply to S4 ground loop help

Hi, I recently switched from pc to mac to run my S4.

When it was running on pc I had the classic intermitant audio drop outs and various bits of lagging, however even when the pc was plugged in there was no ground loop interferance or static.

I got an old second hand black mac and it runs traktor and the S4 beautifully, no audio drop outs, excellent latency and no lag. However when I plug the magsafe charger in I get a deep hum with random audio interferance. Its fine when it's not charging.

The S4 is connected to KRK RP6's via balanced 1/4 leads. I've made the noise a bit better by reducing the gain on the speakers themseleves, but it is far from perfect. I've tried connecting via phono but the interferance is intensified. Also tried plugging into various different sockets to no avail.

As this only happens when the mac is plugged in, not the pc, could this be an error with the mac power supply? Any help is very much appreciated, thanks guys!