Boiler thumps switching from water to heating

For some time now my Baxi boiler has made a loud thumping noise when switching from hot water to heating. It sounds like a valve shutting off too quickly or hitting something hard and the thump echoes around the house.

I've had various suggestions as to what might be causing it - does anyone know the real reason for it? It started about 18 months after I had the boiler installed - up to then, there were no problems.

Chris

What system set up would help i.e Two port valves or a three port valve

Is the bypass open

I believe it's a 3-port valve situated inside the boiler (it's a system boiler).

I'm afraid I have no idea about the by-pass.

Chris