The fact it's night time won't have much to do with it, it's probably once the system is off and cooling down (around bed time). The fact that it's just one radiator means that you only have one to sort out. It could be the rad it could be the pipework it could be the bracket, it's hard to identify unless there when it's doing it.
The fix can even be as simple as to lift/move it on the brackets just a little. If it was me I'd be draining the rad, lifting it off the wall, make sure the brackets are nice and tight and refit and see if it still does it. That would usually rule out all the other bits and pieces and then it would need the rad changed.
If it still does it then it has to be something else, say pipework under the floor etc.