We get a knocking sound under floor what is it caused by.

It sounds like a small case of water hammer.. your pipes may be getting blocked a little, also may not quite supported well enough going through the joists..

Without hearing it, it would be difficult to say. 2 probbable causes. 1. As pipes heat up they expand and due to pressure in pipes if they come in contact with other pipes they can make a light mocking sound. Usually it’s the floorboards rather than the pipes making the sound. 2. Could be dirt/air in the system and could need cleaning out and then balancing and bleeding. Hope that helps with your question.

Possible ball cock worn in header tank

Its water hammer probably pipes not clipped.

Check the TRV, put your had on and turn if it is you will feel the vibration, sometimes the pins stick