I have a number of mains-powered devices that have begun humming (I assume at 50Hz, though I haven't measured it). These include clock radios, a speaker system, a lamp (20W halogen with a transformer before the switch), and a temperature-controlled soldering iron. With some of these devices, it can be really annoying (like the radio and lamp and the speaker that vibrate the floor, and my bed, and make it hard to get to sleep). I assume it's also wasting a small amount of energy. I have tried taking one of the clock radios apart, and cleaned some dust and crap off the PCB (I couldn't see any other obvious problems), and it did reduce the hum a little, but not much, and it started getting worse again.
Is there a common cause for these hums? And is there anything that can be done about it?