system noise following repair? Help!

[bedfri]

I have a conventional CH system with a baxi boiler heating CH and DHW via a tank. It is a sealed system and the boiler is located in an outhouse. The 3 way valve broke (ball valve detached mechanically) The symptom was no CH, DHW working only. Pump and 3 way valve replaced for new, and system now working fine. The pump is set to the mid position. There is a frost stat but it is not cold enough yet! Banging in the pipes happens each time a stat kicks in or the timer switches a system on. If CH or DHW are selected independently, all is quiet.
I have lifted the lid on the 3 way valve to check actuation on switching and all is functioning correctly. I have also bled the system many times in just about every location... No change! Any suggestions please?

 

[Soggy_weetabix]

Banging can be either a sludge probem (kettleing) or a poor circulation problem (or both).

Does system actually work whilst making the noise?

Have you cleared all air locks...air can prove difficult to remove even in sealed systems.

David