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I have a central heating system on a timer, and whenever the timer turns on or off, for around ten / fifteen minutes, some of the radiators in my home make regular clicking noises, kinda hollow-sounding, coming from inside.

I've done some research and it seems that it's something to do with the hot and cold combining and something growing, shrinking, or similar.. is that correct?

I've already bled all of the radiators and there's no excess air in them.

What else can I try?

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I fixed a particularly annoying radiator once by whacking it with a hammer in just the spot the click was coming from. That was probably a bad idea, but I got away with it. – Wayfaring Stranger Mar 26 '13 at 14:22

This is the sound of metal pipes and radiator components expanding and contracting. As they do so, they rub against other fixed parts, like wood framing, brackets, etc. If they stick slightly, you'll hear a popping or clicking sound as they stick and unstick from the friction against this expansion and contraction. So the solution is to locate any points that are making sounds, ensure they have space to freely expand and contract, and eliminate any points that may stick by either changing how it's attached to the structure, or changing the materials to eliminate the friction.

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That's quite a high-level answer, I'd have absolutely no idea how to do any of the things you suggest. How would I know if the area has space to expand and contract? It's inside the radiator, and I'm not at all comfortable with taking it apart (if that's even possible). – lukech Mar 26 '13 at 14:59
@lukech to be fair, it's a pretty high level question, too. Had you posted a photo with an arrow pointing to a specific location the sound is coming from, someone may be able to suggest a more specific fix. From your question, I don't even know what type of radiators you have, so it's impossible to suggest what can and cannot be taken apart. – BMitch Mar 26 '13 at 15:50
They're just standard metal radiators, really. Hot water comes in, they get hot, etc. If you have any specific questions I can try to find out anything that might help. – lukech Mar 26 '13 at 16:13
@lukech Not sure which standard radiator you're referring to, there are quite a few. You mentioned bleeding them, so that rules out a steam radiator. But there are also single piece cast iron types, baseboard styles, and a style similar to a baseboard heater that has small metal fins but a large cover with adjustable vents that I'm aware of. Searching around, there are also various wall mounted varieties. The specific question is where is the sound coming from. – BMitch Mar 26 '13 at 16:55
There's nothing adjustable about it except the valve and the temperature sensors. I think it's a double convector, if that helps. – lukech Mar 26 '13 at 17:28

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