Why does my pump get noisy when rads are turned off?

[MANDATE]

I had a valve that went fully across to the CH only position, but it still did not fire the boiler as there was no power output from the orange wire of the valve. This was due to a faulty microswitch within the actuator head.
A replacement head cured the problem.

 

[DavidJW]

From what you have written you may be wired as a Honeywell Sundial W Plan. This needs to have HW switched on to make the heating work.

Info on the Sundial plans can be found at:

http://content.honeywell.com/uk/homes/catalogu.htm

If you click on "Section 3 - Sundial Plans" in the centre of the page it will bring up the four available. You can ignore the C plan and the S plan.

The W plan does not require a valve with an orange wire so, if your pump does not have one, this is a give away. But if it does have an orange wire then your system is probably a Y plan. In which case either the actuator is faulty or the wrong type of timeswitch has been installed.

If you still have the timeswitch instructions, check if it mentions a switch (usually on the back) which can be set to 10 or 16. W plan (no orange wire) needs to be on 10. Y plan (orange wire) needs to be on 16.

Of course, if you are quite happy wih the way it works, then there is no need to go delving In any case this does not solve your noise problem, the answer to which is a bypass valve.