I have an external USB Sound Blaster. I started to hear a clicking noise after I setup my computer. I recently moved to a new place so I had changed my setup around a little bit. I noticed that my wirless router was causing the clicking noise. My router used to be in a separate room, but since the move I had it in the same room sitting right next to the Sound Blaster. Moving them apart resolved the problem.
I hope this will help someone before you start replacing things and sending things back.
Amber