Any chance of a low frequency component somehow being embedded into the file or a DC signal? Those can cause pops. Probably a super-longshot on a mixed-in-the-box track unless some faulty/noisy hardware was recorded as part of the mix.

 if that doesn't get it, I'd look for a specific FX that might be causing the problem

 
I followed this advice BRUNDLEFLY and it appears to be related to GUITAR RIG 5, specifically the "heavier" patches
 
I have left plenty of space at the end of the track to let tails etc. die off
 
So why would a GUITAR RIG 5 patch put a "pop" at the start of the exported track (a section way before the clip appears in the actual track)? (there are no "pops" in the original track when played in Sonar X3)
 
I think we are getting close and thank you for all of the help so far!

I can try to replicate it with GR4, assuming GR5 is not VST3...?  If it's reproducible, the Bakers can look into it. Any particular patch I should try that might be found in GR4?

Thanks BRUNDLEFLY
 
The patch I am using that gives this issue is: "Billy Single Grange" (based on the "gratifier")
 
Further experiments have shown that if I export a single track with GR on it I get the pop
 
If I disable the FX (including GR) then there is no pop
 
If I "FREEZE" Guitar Rig and then do the export to WAV I do not get the pop
 
What does SONAR do differently to export a WAV compared to freezing a synth/plug in?
 
 

**Update**
 
At the end of the "mixing down audio" during the export of just the "dirty guitar" track, not only do hear a "pop" but the MASTER bus meters jump at the same time
 
Interestingly, the meters on the guitar track itself, and the guitar bus it goes to, do not jump - it is ONLY the MASTER bus!?!?!
 
Does this help indicate the issue?