Im at the stage were I have a project mixed (all audio tracks, no midi).    I have a marker set at a point a few seconds before the song and have deleted all audio before the song starts, only leaving the grey background, yet I am getting a pop sound at random volume levels each time I hit play from this same starting point. 
 
The problem with this is that it is hitting the reverb causing reverb to trail into the begining of the song.  A simple work around is to start playback early enough so the reverb decays before the song starts, but I woul rather find a real solution rather than a work around.
 
I have noticed if I mute every channel the pop sound does not occur.  I did verify i have no hidden tracks, etc.
 
I ran a bounce-to-tracks and export to try to get a final mix and this pop sound was written to the file. 

Some more info I found.  The pop sound seems to reduce, then vanish if I:

1. Hit play
2. Hit stop BEFORE any audio plays,
3. Repeat 1 and 2 a couple more times
4. Hit play a final time and it starts without a pop.

Is this by any chance the reverb tail from the end of the song?

If so, all you need to do is to slip-edit a clip - any clip - and extend it's right hand edge to encompass the full decay of the reverb.

a midi clip would be your best bet as it should have no "hidden" data, unlike audio clips which might!

To test Jonesey's hypothesis, close the project, re-open it and hit Play. Do you hear the pop the very first time you play the song? 

If you do, then there is some event buried in there you're missing. If not, it's the remnants of a buffer from the previous play.