I've been trying to get rid of a clicking noise from my pipes recently. This has got worse since installing TRVs (bi-directional), I have rinsed out all the radiators, added inhibitor and drained all the air, no air seems to be getting into the system, the pump is running ok. I have a small bore system but I've gone 15mm on the TRVs also my house-extension is all 15mm, been a pain on balancing but seems ok. I have checked the pipe runs and managed most of the clicking by using pipe lagging but still have some clicking noise from these, thought about fitting plastic but a little close to the boiler but it is a clicking I can live with, However the big noise is a popping deep noise which I can't locate, I figure it must be from the boiler itself. I have noticed that the feed pipe from the F&E tank gets hot from about 2metres from the boiler but the expansion pipe remains cold. I have checked the F&E tank and it seems a bit stagnant and no water flows from the expansion pipe and the tank is very cold. Any help would be appreciated.