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On Friday, 6 January 2017 19:39:53 UTC, wrote:

Clarification - I should have made clear that the water hammer (when it
occurs in a nearly-fully-heated cylinder) is not the few-Hz bang-bang-bang,
but more more like more-or-less continuous-tone vibration noise that
randomly rises and falls in loudness, randomly dropping to barely audible
for a few seconds only to increase again, and continue until the boiler
finally cuts out.

Is there anything as simple as a ball-cock in a cold-water header tank in
your system? I only ask because I had just such a water-hammer sound as you
describe, like a low cello note. This was just after I repaired the plastic
ball which had got half full of water, but was now so light it vibrated. I
cured it by clipping a submerged metal plate to the arm of the ball cock by
a clothes peg to act as a damper.
--
Dave W