Standby or active?
If standby, it's probably pulsing infrequently to maintain voltage. The pitch can vary with mains supply voltage and will vary with load. Not much you can do here: the noise is likely due to magnetostriction in the core. That is, the core itself is straining due to applied magnetic field. The core is quite stiff, so it's not like you can keep it from straining by putting rubbery goop on it. About all you can do is add padding to absorb the sound (which may however still involve rubbery goop, say to dampen the PCB). Be careful with padding, so as not to block airways.
If active, it may still be the above, if not on the main supply(ies) then on an aux supply or something perhaps. Otherwise, it may be poor compensation, which may suggest aging electrolytics, or poor component choice in the first place.
Heh, also, if it has powered speakers internal or attached, those could be squealing, for similar or different reasons.
Tim