How can I stop my radiators "clicking" when they're turned on or off?
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.