I notice that whenever I do two or more exports in succession, the audio buffers don't clear at the end of each export. As a result, there will always be a bit of unwanted noise at the start of each subsequent export (unless I remember to do something else such as a short playback between each export).

Is this a Sonar issue, or does it have more to do with audio drivers?

Hi. I remember having this on earlier Audio Interface, so I believe that it is driver related. Try resetting the Audio / MIDI engine between exports. 
All the best.

Zargg71
...Try resetting the Audio / MIDI engine between exports. 

I guess that's probably the simplest solution.