What is Causing These Clicks and Pops?

I am noticing that when I go to bounce, I will hear short click/pop just as the bounce begins. It shows up in the final mix. I can play the song without bouncing and sometimes it's there, sometimes it isn't. It most certainly is not audio in the tracks themselves. Yesterday, I hit spacebar to play, and it was fine. I stopped, went to the start, hit spacebar and it popped. It was like every other time I hit play, it had the anomalie.

I am using the RME Fireface 800 on internal sync and nothing else is locked to the Mac.

Any ideas? It appears reasonably random, doesn't happen EVERY time I bounce, but enough to concern me.

Thanks
Tom