Update - solution found
All, thanks for your advice and helpful comments (you can't believe how delighted I am that someone recognised my Bristol 412).
I found then solution myself in the end. There is a small transformer on the circuit board, at the front on the left. There is a thin metal (?) sheet glued on top of it. Not sure what it's for, but it slides off periodically and when it does it touches both the top of the tranny and the main board and vibrates. I've replaced it and all is quiet now.
I have also been prompted to take mains power far more seriously and installed a 3000VA balanced transformer which has added additional clarity.
Kind regards
Andrew
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