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

So is it plausible that a HD1 could be spinning at 120 Hz and HD2 at
121 Hz (7260 rpm) to cause such a 'beat'?

Maybe, but you don't need to know. I've given you two ways to tackle it.
Try them.
--
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