I have a Rehau underfloor heating system on the ground floor only of my house. Every so often, the manifold makes a knocking/thumping noise. When I look at the flow meters I can see the one that's making the noise and the little ball in the flow meter is bouncing up and down (so the water level is bouncing up and down, obviously). Could this be air in the system? It only does it when there is demand on that particular loop (the hall, so there's often demand as the temp changes due to the front door being opened). If I turn the pump speed down it simply changes the tempo of the thumping! Any ideas? The flow meters are beginning to look opaque so I guess the system needs cleaning out anyway, but the thumping has occurred on and off since the system was installed (now 5 years old).