Adam-the-Kiwi presented the following explanation :
Thanks Dave - I'll give that a try (if I can work out how to do it)...

Interestingly, a hefty thump on the radiator in question has changed
the intensity of the buzzing.

That sounds as if it could be a TRV or valve vibrating.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk