Water hammer Armitage Shanks/Ideal Standard dual flush?

Having a problem with water hammer on an Armitage Shanks button dual flush cistern mechanism. Its the same mechanism for Ideal Standard (as the same company!).
What is happening is shortly after the cistern has been flushed there's a very load thump from within the cistern before it starts to refill. This water hammer thump is so severe that it can be clearly heard throughout the house and the vibration felt through the timber floor. Apart from this the mechanism is working correctly.
It seems like a damper or something is sticking but haven't been able to identify what is wrong. Has anyone any ideas what is causing this water hammer and how to resolve it without replacing the whole mechanism?