Why do my radiators occasionally hum or bang like thunder?

My boiler isn't particularly old, but every now and then we seem to go through a period where there is a sudden onset of loud humming in the radiators, this is accompanied with mive vibration, we can usually stop it by twisting the knobs on the loudest radiators but sometimes it stops for a few seconds or minutes and then starts again. and not always in the same radiator, it seems to travel around the house, its not a big house but it is a real pain in the bum trying to sort it out, do you know what it is, or what I can do to resolve the problem, all advice welcomed.,