hi , ive recently started to get banging noises in central heating pipes. the far away rad in house is much colder than the others so i tried turning the other rads down to see if this works and no matter what one i turn down the noise starts,tried bleeding the colder rad,(no air) Also the old shower has been replaced and i think the heating got switched on after the the mains supply stop cock was shut off which feeds both boiler & shower.Is this an air in the system problem ? if so how do i release the trapped air?HELP PLZ 
