Vibrating what now?

[squeefulfish]

This is not the sort of question that I normally have to ask at this hour on a Sunday evening when I'm perfectly sober, but is it normal for radiators to vibrate?

No, really, it's vibrating. Not so much that I can hear it, but when I touch it it feels all... well... it feels like... it's vibrating. Damnit, there's no other way I can describe it!