I'm getting a sort of 'beat' effect from my PC. It seems to have been
triggered (or maybe just increased to a level at which I've become
aware of it) by adding a new (third) HD.

I'm at my desk, and the PC is on the carpet 3 feet away, at the side
of the desk. I can just hear this beat from here, behind the familiar
high frequency fan and HD spinning sound. Kneeling on the floor with
my ear by the PC case, I can hear the beat distinctly, at about 1
cycle per second. But my real gripe is that directly below, downstairs
in the lounge, it's surprisingly distinct. By no means deafening, but
it is intrusive. I'm aware of it even while watching TV. This is
through carpet, underlay and 2 layers of plasterboard.

I did also have mechanical noise from the fan (which itself may have
been due to multiple causes, such as bearings, dust and one loose
screw). But I've fixed that, and am left with this pesky beat. It
doesn't appear to be anything I can stop by touching any PC panels, or
even holding the fan itself. So I assume it's some sort of beat in the
air itself, caused by the rpm speeds of various components being very
close together? Rather like 'heterodyning' in radio terms?

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?

--
Terry Pinnell
Hobbyist, West Sussex, UK