"Brian Gaff" wrote in message ...

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


Oh yes I'm sure that you are right - it's classic feedback oscillation. And
I'm equally sure the solution is to increase the damping on the valve, the
problem is how, as it's a sealed module as far as I can tell.

AWEM