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Thanks. I think it may be a relay too. It sounds as though something is trying to repeatedly switch itself on to amplify sound. I doubt that any foreign object has got into the unit as I only had the cover off for a few minutes to check the internal fuses connected to the circuit board (which were all okay).
After your tip-off I used Youtube to check for similar faults and yes, it does seem from looking at uploaded videos that it may be the input relay.
I just need to know what it looks like now so I can replace it. :-)