Feedback From PC Sound System

I have intermittent feedback noise coming from my speakers. It does not matter if Sound Card is in use or idle the noise persists. I have swapped Sound Cards to no avail; I have plugged system into different wall socket; used a noise filter between wall socket and surge protector and visa versa; I have swapped surge protectors; I have switched subwoofer; I am at my wits end. Help!