Rattling sound from 3 way valve or pump.

Hi,

I have resently installed a new 3 port valve (Siemens 22MM 3 Port Mid Position Valve CMV322) but there seems to be a rattling sound coming from it as if air is trapped inside. It only happens when the hot water circiut is on and when the central heating circuit comes on it dissappears. I have bled the system quite a few times (inc. radiators), there is a bleed pipe on the central heating side just after the valve but not on the hot water side. I have bled the pump too which is just below the valve, also new. There are no banging or other sounds associated with air and no leaks anywhere. If I have disturbed a joint there are no leaks.

It is a vented system with a small tank in the loft. The vent pipe is on the suction to the pump.
I have also drained the system down, added a desludging and descalent liquid, and treated with Sentinal X100.
Regards
Steve.

Try closing the pump gate valves by a quarter of a turn or so. They sometimes rattle.

I had exactly the same problem - a rattle from what I thought was the midway valve. I had changed this valve because it had stopped working and as the rattle appeared straight after valve replacement, naturally thought I it was the valve.
Then I read Oilman's thread and hey presto - turning the pump isolation valves cured the rattle. Wasn't the mid way valve after all. Top marks to Oilman - Thanks

Hi just found after posting something very similar and only started happening when pump head was changed and isolation values were turned off. Though one of them didn't seem to turn off even though fully turned
After changed pump head they were turned back on, could one of the isolation valves be faulty or not fully open