On Friday, 6 January 2017 23:52:23 UTC, John Rumm wrote:
On 06/01/2017 21:46, wrote:

2) The noise issue has (I'm told) re-occurred from time-to-time over
the 12 years (approx) of usage. Allegedly no problems in the early
years, but has come and gone spontaneously after that.

3) Cylinder stat set to its mid-point and what I would describe as a
normal domestic HW temperature at the tap (sorry, no thermometer
immediately available).

4) Circulator pump is built into the boiler (Ideal icos) and is
non-adjustable single-speed.

Could it actually be bearing noise or vibration from the pump itself
that is just "ringing" the coil in the cylinder?


--
Cheers,

John.

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Yes - because of the pattern of vibration I had indeed wondered about 'ringing' of the coil.

One theory I entertained was that as the tank coils got hotter that they were expanding and becoming loose in whatever restrains them (I've never sawn a megaflow in half to determine all its secrets).

However Tim+ suggests that it relates to the bypass valve, and that would rather fit with an ill-defined pattern of occurrence of noise dependent on the state of each individual TRV.

Perhaps a combination of the two - a coil prone to resonance, and a bypass valve issue that triggers it?