I think it's fairly common for speakers to pop on computer startup and shutdown, even if the speakers are always powered. Two of my last three computers, and one of my parents' two computers pop. I've always thought it was a speaker and/or sound card thing, as I moved my decent 2.1 speakers between the two that popped. The only action I did was turn down the volume before shutting down, if anything. Bugs me less than the windows startup sound (I disable that).