AOpen AK73 Pro with onboard sound and firewire

...never mind...

It sounds like some kind of interrupt problem. A PC can sometimes get into some weird state where each interrupt generates an audible click in the sound card. Perhaps emplode using the USB controller changes something that makes it stop. I can't think of anything that sounds less farfetched...

You could try turning off channels in the mixer to determine which channel is producing the noise. Perhaps it's one you don't need. Make sure you check all the boxes under Options/Properties in the mixer so you're actually seeing all the channels the mixer has.

/Pepijn