How to stop a glitchy 'popping' noise during playback?

I've noticed this really weird clicky, glitchy popping noise which seems to only happen during playback in iTunes. I've tried watching DVDs, youtube videos, streaming music on spotify etc etc. But when I'm in iTunes, and I'm playing my music - that noise is there. It's hard to describe, but it's almost like click and pop at the same time - and happens every couple of seconds, but ONLY during playing songs.

I've reinstalled iTunes, and restarted my iMac etc etc - so I'm really not sure what is up with it.

I'm using the latest beta of iTunes too, not sure if that's any help (because it didn't do it when I updated either, it's only recent in the past 2/3 days)