Presumably it's heating all night and buzzing due to the expansion
of the water.
If acceptable, modifying the timer so that the cylinder doesn't heat
up during your sleeping hours might be a solution.
If you are really desperate in the middle of the night - you might
find that you can stop most of the buzzing by running off 2 to 3 litres
of hot water from a tap.
This could well be enough to ease off most of the excess pressure
caused by the expansion of the water when heated up.
If however your problen is not just noise and you are getting a fair
amount of water coming out of the pressure relief valve and draining
away through the tundish, then it might be worth getting someone
to check out the expansion cylinder.
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