Audio System Diagnostic Help Please



I'm having a problem with my audio system and need some assistance with a diagnosis. Please be patient with me as I really don't know a lot on the topic. But I know from reading some posts that there are experts out there :)

It's a little bit hard to describe what I'm hearing. It sounds like a very high pitched vibration. There might be better words for it but I'm at a loss.

I have the "professional" radio. Door speakers and tweeters. Rear speakers on the back deck. Not sure about a sub.

When I isolate each corner I can still hear the sound.
The sound is there whether from the radio or CD.
No codes at present when I check with my BT scanner.

Any ideas? Amp? Control head?

I took it to a audio place and he said it would be just trial and error to find the part. There has to be a way to narrow it down I'd think.