This is probably electrical interference or static via the outlet that the Mac and/or speakers are plugged into. In my case, whenever the attic fan kicks off, I get a pop through the speakers attached to my '09 Mini, because they're all on the same electrical circuit. The best thing to do is try to find and isolate the source of the pops that are on the same circuit, such as an appliance or other electrical unit that may regularly switch on or off. There are also outlet filters available to help eliminate this.