Click/Pop in exported file - not there in realtime playback

I’m having a problem exporting a finished mix. When I play the project in real time it sounds perfect. But when I export the mix there is a click or pop at the beginning that is feeding my echo and reverb FX - creating echos and noise at the beginning of the song where it should be silent. These sounds are NOT there in real time playback.

I also noticed that sometimes when I open and close certain dialog windows I am getting a click or pop that is bouncing around afterwards in my echo plugins.

What could be causing this and why is my exported file different than the real time playback?

BTW - it’s intermittent. If I export the track over and over again eventually I get a clean version that does not have the “junk” in it at the beginning. I don’t know what, if anything, is different between these exporting attempts. VERY FRUSTRATING.

I’m on CB 6.0.7 on Win 7, 32bit, 4GB RAM