In an earlier contribution to this discussion,
John Stumbles wrote:

You may be able to alleviate this by changing the pump to a Grundfos
Alpha.

These automatically back off their power as the flow resistance
increases, which I suspect here is the reason for the noise.

Is there any danger of them backing off so much that there's
insufficient flow to cool the boiler during pump over-run conditions?

Yes, I have heard that caveat (though I haven't come across that
situation in practice, but then I've come across vanishingly few
Alphas)

Yes, I wonder how well they work with automatic by-pass valves. AIUI, an
automatic by-pass only opens on the pressure rise resulting from low flow
through the rads when all the TRVs are closed. If the Alpha backs off so
that the pressure *doesn't* rise, the by-pass may not open and the boiler
may overheat.
--
Cheers,
Set Square
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