My first thought was a whining DC-DC converter, but I think they are linear power supplies, so probably not.  Also, I thought you meant signal noise.

I guess it could be a buzzing transformer, but I wasn't aware that could give a high pitched sound.

I don't know if this helps, but there appear to have been three different versions of the power supply:

https://www.muffwiggler.com/forum/viewtopic.php t=27314&sid=88d7c9202a1c15c9911203676478376e

If it's a mechanical noise, you might still get it with all modules unplugged, or failing that with just a dummy load, e.g. a small torch that can take 12V, connected to a bus board.  If so, it would have to be the power supply.

- Andy