Liquid Cooling Noises

So I have been using a Swiftech liquid kit to cool my CPU for about 6 months now, and it's great, but there's something that's been bothering me for awhile.

Ever since I rid myself of that miserable nvidia board; swapped the kit into a new setup, there has been this kind of "popping" sound in the radiator that happens quite often. At first I thought it was just a bubble trapped in there somewhere but it's been doing it for a long time and hasn't gotten any better. I've tried the standard rocking/tipping method of trying to free a stubborn bubble many times but that hasn't changed anything. The flow is good and there are no leaks so I can't debunk that noise. Sometimes it's louder than usual and sometimes 10 minutes will pass before it happens again. It doesn't seem to be causing any physical damage (though it is a little annoying to the ear) so I've just been ignoring it. Any ideas what it could be?