[Benjamin]

If it has gone away, it will be difficult to track down. If it happens again, I would maybe try swapping left and right dash speakers to see if the sound follows the speaker or stays on the right. At very least it would rule out the speaker. Otherwise, since the PCM is off, it would have to be an induced hum from something else that has wires in the same harness as the right speaker. I'm guessing that you haven't changed anything in the harness and since it wasn't happening before recently, it's not just a "re-route the wire" problem, something is producing a hum that isn't normal.

Do you have a durametric tester? It allows you to turn on different electronic components (fans, etc) one by one and you could see if there is something that reproduces the sound.

Good luck, and please keep us posted.