Alex Threlfall wrote:
In article .com,
says...
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.
[...]
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.
[...]
Probably the PSU, i've had similar things happen to mine and it drove me
potty(er) too 
When I change to another window or change significantly what is in the
currently displayed window, there is a noticable increase in volume
aabout a second or two afte the changer. I guess a graphics card
(Radion 128-MB something) does draw significant current. So could point
at power supply.
I'll report back as soon as I get hold of a stethoscope and do a peoper
investigation.
Thanks all,
Jon C.