Sound Bug?

Ok, so i have a really weird problem. I have no idea what is causing it either. When i watch some flash videos (and only flash videos, music works fine), the speakers hum really loud, (almost like a modem connecting ish), make a electronic beep/pop noise, then go silent. Then 30 seconds later the sound comes back.

I have the headphone jack on my computer going to a mixer which outputs to two monitors. I know its not a problem with the speakers themselvs… but maybe its the sound card? it keeps scaring me half to death… its really loud.

anyone else ever heard of a similar problem? Its the exact same hum/ pop noise every time.

Edit: I’m running vista 64 if that helps… and it seems to only happen when watching videos on youtube. I have a creative soundblaster x-fi