Why your radiator is loud, in one paragraph
And three paragraphs on what to do about it.
Here is the paragraph. In a steam system, the banging is almost always water trapped where only steam should be: condensed water pools in a sagging pipe or a tilted radiator, arriving steam slams into it, and the collision rings through the metal like a struck pipe — because that is what it is. In hot-water systems, the gentler ticking and gurgling is usually air that has worked its way in, plus ordinary metal expanding as it heats.
The two fixes worth trying yourself
First, the tilt. A steam radiator should lean slightly — a checker under two feet will do it — back toward the boiler in a one-pipe system, so condensed water drains home instead of pooling. Five minutes with a level often retires a bang that has been in the family for years.
Second, the air. Hot-water radiators have a small bleed valve at the top. With the system off and cool, a quarter-turn with the key (or a flathead) lets trapped air hiss out until water follows. Catch it with a cup. The gurgle usually leaves with the air.
What you should not do is live with it out of politeness to the building. A knocking radiator is not a personality trait. It is a maintenance item with a five-minute fix attached, and winter is long.