Just an aside (not necessarily any direct bearing on what you've said):
In the depths of winter I heard my machine making the most awful juddering
rattling sound - or so I thought. I became concerned to the extent that I
turned the machine off. And then it was obvious: the noise wasn't caused
by the computer, but by a bus outside struggling in the cold weather.
What this taught me: it can be very hard to place the source of a sound.