Also, the arcing caused by flipping switches in the houses electrical wiring (like an overhead light) will generate bursts of EMI. As has been said before, since it happens when your PC is off and the speakers are on it is an indication that the cheap amps high impedance inputs are recieving EMI. It will be nearly impossible to fix the already broken design. If this is your problem then there is an opportunity for a speaker upgrade.
RichPLS, a failing bearing will most certainly cause a brushed DC motor to emit more EMI. I don't think it will have as noticeable an effect in the brushless DC type motors used in PC cooling fans, ie the coil current may double but that doesn't necessarily indicate there is much more EMI energy. PS these are just guesses about your interesting failing bearing question, and I wouldn't want to quote me or try taking what I said to the bank.