Actually, the CPU modulates several ampere of D.C. current depending
on usage (current consumption is lower in the HLT state, hence the CPU
runs cooler...) If the power supply is bad, these variations can
easily be passed on as voltage drops to other components in the
system, and as someone else points out, hard drives are blatantly
microphonic.
cat /dev/zero does exactly one thing: it keeps the CPU running 100% of
the time. Hence, if that quiets the machine, it is a pretty safe bet
that the noise heard is derived from the varying current consuption of
the CPU.
-hpa
-- Not speaking for Transmeta in any shape, way, or form.