These popping noises happen because the flow to the valve is going backwards. It is not always easy to see the supply lines coming from the floor or the wall.
A valve must always be supplied from beneath the flapper, which means from the supplying line.
To address this malfunction, COMAP redesigned the flappers in its thermostatic valves in 2013. The flapper is reversible and functions silently, equally well in one direction as in the other.