Probably because the sound can not keep up with the video. Also might be that the "peaks" in the audio part are to high, causing a pop...
Might want to try rendering a small portion of the 'popping' part to an AVI-DV file and playing back 'that rendered file' via Window's Media player to see/hear if there is still a popping noise.
Vegas defaults to 44.1k audio, DV audio is 48k. Maybe your computer is stumbling while converting on the fly to 44.1. Try setting your prefrences to 48k audio if you haven't already done so.
File\prefrences\Audio