Ok so no guarantees this will work, but it did for me and I knows it's very late but hopefully it will help someone else with a similar problem.
I had the same problem and as clmrt5000 said a very long time ago there could be an issue with a ground loop, although this is likely to give a high pitched buzzing sound.
This can easily be fixed by adding a switch to disconnect the mains earth from the back plate, or you can just unscrew the earth and tape it up, this fixed my ground loop, (some models already have a ground lift switch)
As for the "boiling" sound I also got a saw tooth wave sound (like a strange squeak or beep that changed frequency) this is caused by faulty components, and thanks to Frank Johnson (http://lug.demon.co.uk/kefrepair/?p=60
) who suggested that you can simply and easily replace 3 capacitors on the main amplifier board (c58 - 47uF 63v or 100v) + (c52 & c53 - 220uF 25v) but stand them away from the board on the length of the capacitor leads to prevent them from absorbing heat from surrounding hot components (with heat shrink tubing to prevent shorting)
Now my sub works as good as new, apparently these 3 capacitors can cause all kinds of problems it's a common fault, so if you are handy with a soldering iron, or know someone who is you could save yourself a lot of money by repairing you faulty KEF sub.
p.s. if you have ANY of the KEF PSW subwoofers they all have similar issues, you can find a guide to fixing them at the link above, Frank is a brilliant and very helpful man.