Very strange noise on radiator

Tim_23

Hi there. I have a problem with two of the radiators in our house. The two radiators, which have new air vents and are properly pitched, are connected together under the floor (when one radiator is bumped or the main valve is closed, the other will also move - and the sounds telescope between them both)

The noise that I hear is not a loud water hammer. It is a more mild sound like if there was a small polished rock rolling around in the radiator for a brief moment. And importantly, the noise is cyclic. Probably with a period of 3-5 seconds. There will be a growing gurgling and then the "rock" noise (like just a few "tumbles of the rock" if you can imagine that. Then repeat. This happens after at least one of the radiators is warm. If one main valve is shut, the noise persits. If both main valves are shut, the noise will stop. Initially it seemed it was the main valve seat, but if you shut only one of the valves (either) the noise will continue.

So if it happens to be some sort of wierd unstable flow issue at the T between the two radiators, the easy fix is a pair of ear plugs. But if someone has encountered a similar problem, or can think of an experiment to try to determine the problem, let me know. I'm all ears.

Thanks,
Tim