Andrew Mawson wrote:
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.

I had the same problem but now it's cured, try these first
http://snipurl.com/26mcwe1