No, it's not the fan, it's the oscillator I'm hearing, but I don't know
what components are doing the vibrating. The fan fires up occasionally,
but that noise is entirely different.

I've heard switching PSUs begin to hiss when the electrolytic filter
capacitors start to fail. These caps are in a stressful operating
environment - high ripple currents, and sometimes high temperatures.
They can dry out or leak (this is a particular problem with a whole
batch of Taiwanese-made caps which were manufactured with a defective
electrolyte formula), their ESR increases, and the circuit becomes
audibly noisy. I don't know whether the noise is direct microphonics
from the caps, or from the toroids.

Consider swapping out any suspect 'lytics in the supply with new ones,
preferably ones rated for switching-supply operation at 105 degree C.

--
Dave Platt AE6EO
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