central heating pump noisy on overrun

[tangent1]

i have had various issues with my central heating of late and solved them all bar one.
My heating works fine and quietly until its time to turn off and then the pump starts to to make a bit of a a racket. why would the pump only make a noise on overrun?
I have tried to bleed the pump but it doesn't help. Pump is mounted horizontally and on its side so that doesn't help (it cant go any other way due to walls and space etc). Can anyone help?

 

[seco services]

what systems it on y, s plan etc?

 

[tangent1]

y plan

 

[Agile]

Do you have two two port valves?

Tony

 

[tangent1]

yes... i have two two port valves.

 

[seco services]

its not a y plan then thats a s plan.

 

[tangent1]

sorry... s plan it is.... i have an update on the problem. I turned the pump up to speed 3 when it was on overrun and noise got worse initially then quietened down a great deal. I then turned it back down to 2 and the noise is almost completely gone... just a few clicks.... does this help with diagnosis?

 

[stardanny]

Is a bypass fitted? something like a small gate valve or a valve with red knob on it.

This is for letting water through when both 2 port valves are closed to ensure water flowing when pump overrun run.

Dan.

 

[tangent1]

the only gate valve i can see is the one on the pipe going to feed tank in loft. cant see any others.