Big Bangs

I have a 3yr old potterton profile(40eL) gas boiler feeding 7 rads. Recently had the top rad moved in my bathroom. When plumber left system was quieter than it had ever been. Recently however,after the boiler has been running circa 1-2hrs there then follows a series of quick taps in the ceiling I get a very load creaking sound followed by violent shaking of the pipes between boiler and hot water cistern and what seems to be a rush of bubbles in the pipes followed by two large bangs and then relative calm again. I have bled the system and turned down the pump setting from 3 to 2 to 1 (on advice of the plumber)to no avail. Recently the good weather has meant less time for the pump and heat and this has reduced the bleeding needed to the top radiator and the number of instances of banging. Please can someone give me an idea of how to stop getting air in the system/stop the banging

sounds as if your boiler is litrally boiling . I would check the boiler thermostat :

At a guess I would say that your rad's have thermostatic valves fitted and the one you have removed was used as the uncontrolled(without a trv). When they shut down the uncontrolled rad would allow a circuit for the water to flow.

If this is the case remove a valve head(usually unscrew) and should be ok. Put your pump speed back to its original setting.
Let us know how you get on.

Ta very much.

I'd just like to thank kevplumb and BDL for their replies. Especially BDL who I think may have solved my problem. I'm sitting with the TRV I removed off the new top rad, turned the pump back up and so far the huge creaking and banging have not returned. I'll let you know if things change