You should be able to find the main cable coming out of the power supply
that feeds the rest of the computer's components.  Disconnect this while the
computer is shut off and then turn it on.  At this point, nothing else is
powered in the box so if the noise resumes, it is the power supply.  When my
neighbor took his down to the computer store to replace it, we made them
plug in the new one with me standing right there with a short-wave radio on
80 meters and listened for any noise from the new one.  It was quiet but
when he plugged in the old one, the noise came on big time.  Very dramatic
demonstration.

Good luck and 73.
Kevin, K7RX :)