I've got a two bed maisonette, the radiator in the smaller bedroom of the two has a noise when the heating is in operation. It sounds as if there are pebbles swirling around in it. All the radiators in the system are heating up correctly i.e. uniformly with no trapped air including the culprit. It's about twelve years since the current boiler was installed and a new hot water tank was put in at the time. Everything is working properly but as I use the room as a home office it's so loud it's becoming a distraction. Any help appreciated, thanks.