Ahh yes,
So this is a little complicated to explain, but here goes.
I have seen this issue 100s of times on 100's of different pieces of gear.
You are looking for a coil or transformer that is "coming loose inside" the HIGH FREQ that it operates on/with is mechanically vibrating the coils or windings and emitting a sound.
The trick I use is a stethoscope, you know like what a doctor uses to listen to your heart... I take the end off so it's just a plastic tube... stick it down next to all the coils and transformers... everywhere actually until you find the loudest part of the unit.
Chances are it's going to be a stand up coil, with a plastic core looking thing... I think they were usually some kind of "tune-able" coil, then they get sealed.
Once you find the culprit, remove it from the PCB and soak it in Lacquer. I have a can of paint on lacquer that I just soak them in overnight. Then let it dry for a few days or whatever... then put it back in and try it!
