"hupjack" hupjack_nospamtoday_yahoo.com wrote:

I've got your joe-average ATX case and I can hear a faint hissing coming
from the power supply even after the computer is shut down. I can kill the
sound by flipping the switch on the back of the power supply (which kills
power into the psu). I know "faint hissing" isn't super descriptive, but
can anybody tell me whether I should be afraid of this psu? Is in the
process of frying itself? its capacitor? or some other part that I'd rather
not fry? Or is it no big deal and some psus make a bit of noise? I don't
have any past experience with this hardware, so I'm afraid I can't say if it
used to be quieter or not.

Switch mode power supplies can make noise. It just the magnetic components
(and sometimes capacitors) rattling. Hearing it depends on the switching
frequency which nowadays is most likely ultrasonic, however, a lightly
loaded switcher may have a much lower and possibly erratic frequency or
operate in audible bursts.

Modern supplies will offer a couple of amps at 5v for standby power, this
will be a switcher and will be on all the time mains is applied. When your
PC is off it will be lightly loaded so it is probably what you can hear.

If your PSU has a standby supply (check the label) and when 'off' it
doesn't smell or stay more than vaguely warm I would not worry about it.