My guess is that it is the internal power supply...

It uses multiple voltage rails that are done with switching converters, and sometimes the inductors used in those converters can become noisy due to either coil coming loose from the ferrite (glue giving up), or the actual ferrite has cracked or the ferrite halves have come apart (glue giving up).

You should hear a difference if you try to tap the unit. Once the coil/ferrite position changes the noise level should change. If that is the case, it seem 99% correct diagnosis...