why is my fridge/freezer making a loud rumbling sound?

The cause of the noise is almost certainly the compressor motor vibrating. It, or the associated pipework, may just be rubbing against something. You may find, if you pull the appliance out from its location and slide it back, the noise will stop anyway. But check to see if anything at the rear of the appliance is rubbing together. It could also be the fact that the appliance isn't level. One final thing it could be, is that the motor bearings are becoming badly worn. In this case, there isn't much you can do about it, as the motor/compressor is "sealed"..

Dave E
June 2009