How old are your USB devices you are using? The sound might not be anything or it could be one of the USB devices is getting old. I could be wrong.
I had a problem with my gpu last year and sometimes when monitor turned off, I would hear that sound too. I spent a few months trying to figure out GPU problem and I think I fixed it by replacing my mouse. replace mouse, GPU comes good again?
I never found anything to back it up but we threw everything including a fresh install and stress testing, at problem and nothing we tried would fix it.
No USB hub on my monitor.