I have recently had wet underfloor heating and a new combi boiler
installed. The flow and return go from the boiler through a
thermostatic mixing valve to the 7 port manifold for the UFH. There is
a pump within the combi and I also have a pump on the flow pipe between
the TMV and the manifold. I only have 3 ports on the manifold being
used at the moment (ground floor only).

I am experiencing loud noises (rumbling/vibration) from the boiler at
various times when the UFH is on (don't get the noise when drawing
domestic hot water).

The combi boiler instructions (Glow-Worm 38cxi) state that there is an
internal automatic bypass valve fitted and open one half turn.
However, the diagram below it shows an optional ("if required")
external bypass. It also states that a minumum flow rate of 1289 l/h
(20 l/min) should be maintained at all times.

Do you think the boiler noise I am experiencing is down to me requiring
another bypass in the system. Or should I try adjusting the internal
bypass valve?

I really hope I have made sense!

thanks in advance for your help.

J