|Well, I guess it either has to be a virus or some sort of radio breakthrough. Reformat should cure the former. Nothing lost in trying MalwareBytes freebie if you haven't done so already (it often finds what AV's miss).|
The only other thing I can think of to try and isolate it, is to go to your sound settings and see if muting anything in particular makes a difference.
Always pop back and let us know the outcome - thanks