I read in sci.electronics.design that Terry Pinnell
wrote (in
) about 'PC CPU fan', on
Mon, 3 Oct 2005:

I'm not sure what the best approach is. Presumably I can take the beat
frequency out of this apparently sensitive LF region by lowering the
CPU fan speed rpm a bit? I could wire a couple of series diodes in the
supply. Maybe as an initial step I should buy a new 60mm CPU fan, which
is surely unlikely to be similar in rpm speed to the present one.

Any advice would be appreciated please. Anyone else had similar
behaviour?

Yes, with my old DIY computer with two hard drives.

What is your computer standing on? If it's rigidly connected to the
building structure, the beating frequencies will get into the structure
and appear, amplified, in all sorts of strange places, as you found.

Try standing the computer on a THICK (think 75 mm) of none-too-soft PU
foam, that doesn't collapse under the weight.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk