My PC (Dell Precision 370, about 14 months old) is emitting a high
pitched tone; I'd say between 8 and 12 KHz but that is a guess; that
guess based on recall of acoustic-coupler modems. Loud enough to be
annoying; hardly loud enough do much about.

It is not the (19 inch flat analogue, not CRT) screen and I have
isolated other items like USB disks.

Reluctantly, I switched the machine off and the noise disappeared. The
machine has been switched off only once before; not green, I agree, but
I don't like switching off critical machines.

A transformer or other power supply part?

There may be a momentary slight modulation of the sound when the screen
changes, hence the initial suspicion of the screen.

Laziness, I agree, but I'm putting off dismantling everything and
opening up the box; and given the frequency, it will be difficult to
pinpoint anyway.

TIA,

Jon C.