Hot Pipe

I have a gravity fed system, 2 tanks in the loft etc.
The radiator I have in the hall is fed, from what I can see, from a down pipe coming through the ceiling that remains very hot all the time, even when the heating is off & all the radiators are cold (though the one in the hall remains slightly warm all the time). The reason I think it is being fed by this pipe is that when i turn the heating on there is a rush of hot water through the pipe that appears to fill this radiator up before any of the other radiators. The hall rad appears to have a return pipe that then comes out of the other end of the radiator & goes back up through the ceiling to feed another rad? It also appears to have branch that goes towards the living room rads.
As I understand it the very hot \\\"feed\\\" pipe that runs through the ceiling also goes across the landing to the immersian tank?

The problem is that this hot pipe \\\"bangs\\\" all through the night when the heating is off.

I have recently bled all the radiators over the last few days & there is no air in any of them, & all of them fire up okay when the heating is on. I have also adjusted both valves at either end of the radiators to see if it will cure the hot pipe problem, but to no avail.

Any ideas? Could it be check valve problem?

Is the immersion heater on?

what sort a boiler we on sounds an ark of a system

It`ll be the Noah duplex gas fired mk.2 converted to natural gas from town gas ........have a look @ the water level in the small tank in the roof ......or call a plumber (E. Sussex only)

Immersion is on

boiler is: Ideal, E type RS Balance flue gas boiler

water levels in the tank seems fine

I have just spent last few days draining the system down, flushing it with central heating cleaner& changing one of the valves on the hall rad but still have the problem?