Howzit...got problems with air in my central heating pipes. Bled the system to install a new towel radiator,refilled it (pump off),then let the air out the radiators and started it up again. Was all good,now when the heating is on its all good but when i shut it off every now and then it sound like i got a gremlin in the wall. Loud banging...have rebled the radiators..all good but am not to sure how to bleed the pump. Any help????