Combi Boiler Diverter Problem

tonyw101

I have a Biasi Advance Plus 30 which has had a few minor issues of late, one of which was the pump failed and was replaced.
Since its replacement, there is a load thump that can be heard from the boiler and in the central heating pipes when the hot water is switched on and the central heating is active.

The fault has been tracked to the diverter valve which was found when the diverter motor was removed and the valve manually pushed forward with a screwdriver, as would happen when there is a hot water demand.

With pump running, when the valve moves forward to allow the flow through to the hot water heat exchanger, at about half way, the valve gets pulled forward and impacts the back of the manifold or it "flutters" banging on both the manifold and the diverter motor actuator pin. The diverter valve has been replaced in case the spring tension was too soft, this did not rectify the fault.

It seems that there is a pressure differential between the two zones which is causing the valve to move with significant force.
My boiler engineer is totally flummoxed by it and Biasi themselves are none the wiser.
The manifold has been checked for any problems or debris, it is totally clean.