What is causing this radiator to make a lot of noise?

Ok. My housemates are really annoying and insist on having the heating on overnight. We have a spare room that has had some problem with mold and cold so they also insist having the radiator in there on all the time. This room is located just next to mine, with the radiator being annoyingly placed on my bedroom wall.

At night I find it really hard to get to sleep because it makes 5–6 thud/tapping noises (grouped) every 30 seconds. What could be causing this?

I have thought of a few factors – that it needs bleeding. But also the room in there is really small with a wardrobe being placed in front of the radiator so I thought maybe that may have stopped the heat circulating and thus caused problems?

Also as a side note, sometimes when my radiator is turned on (I turn it off overnight because a) I hate wasting energy and b) I like to snug up in a nice warm bed with cool air outside) I find it also makes my radiator make these noises but not as frequently.

Thanks for any advice.