SemiLiveMusic
I agree with Roel. I often get a random sound just like you explain. And it comes from varying sources. At work I sometimes get it from a radio/boombox thingy. At home on my pc, radio and TV also. I believe it is caused by both cellphones and cordless phones of any type that are in proximity of a reciever/amp/speaker.
I suggest perhaps asking the phone co?
good luck

------------------
Thanks for listening to,
The old Newb