This happens to me as well. I think it has something to do with WAVs and your audio card. I ran into this a while back when using expanded (Installed) RealDrums. When I use the WMA files, I seldom get the pop. If you expanded them, you may Archive them and check the Use WMA files... option in RealDrum settings.

I also get the pop when generating a RealTrack solo. It is annoying, especially on gigs.

I posted something on this a while back asking other BIABers to verify. Oddly, some people had the problem, others did not. Just make sure you have the latest drivers for your sound card. You may want to try ASIO as a driver instead of MME.

Keep us posted if you get rid of it somehow.

Rachael