Some ordinary valves do this, its a kind of hiss with an hf whistle. I
always put it down to the restriction at the valve as its altered causing
turbulance in the water.
Brian

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From the Sofa of Brian Gaff Reply address is active
"Andrew Mawson" wrote in message
...
We've recently had a 'Continental' brand under floor heating system
installed which works very well indeed, however at times it whistles!

The noise emanates from the mixing / temperature setting valve on the
manifold and is at a very high pitch (top of the audio spectrum). If I
slightly adjust the setting, the noise stops, until (presumably) it comes
back to the new set point, when the noise starts up again. Has anyone else
experienced this with their system? I am about to contact the
manufacturers, but would like to be armed with more information if it is
available.

AWEM