Assuming this is a pressed steel rad mounted on wall brackets, one
possibility is that the noise is caused by movement and metal-to-metal
contact between the mounting lugs on the back of the radiator and the
brackets that are screwed to the wall. Plastic spacers are often supplied to
prevent metal-to metal contact but these are not always fitted. If that is
the cause you might get away with spraying WD40 - if that works it more or
less confirms the problem. Otherwise you need to drain the radiator and tilt
it forward/remove and fit plastic spacers or similar to prevent the metals
touching during movement.
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