at the risk of being on topic, have you considered adding some sound deadening to the case as well?
I'd try sitting the case on something (a piece of high density foam or polystyrene sheet, or even corrugated cardboard) and probably lining the sides as well (they make great drums!!)
Good ideas but all this is happening inside an Antec case that has padded sides already. I've also tried putting the case on top of some padding but the machine is so heavy (around 3 stone, or 20 kg) that there isn't anything sufficient to put under it - maybe a bed matress (we can now discuss bed/pc mods to go with the toilet mods).
Anyway, the annoying humming noise has gone - definitely. It just isn't a silent machine overall at the moment so, when the extension cables arrive, I will be putting the machine in the other room (neighbour's toilet habits permitting).
For anyone experiencing a similar annoying hard drive hum, this is the material I've used: http://en.wikipedia....rg/wiki/Sleeping_pad
I've cut several small rectangles out from the foam so that each hard drive rests on four of these layers. This is something I shall do in future (as it works better than anything else so far) - but it requires a more secure way of restraining the drives. It's fine whilst the machine is stationary, but if I were to forget about the hd foam bed setup I could easily have hard drives smashing around inside the case when I try to carry it somewhere (I've stuck a post-it note to the side of the case to remind me about the unstable contents - yes, I am likely to forget otherwise).