PlLEASE NEVER bleed your rads while the boiler is on or the pump is running. Make sure everything is turned off before you start bleeding radiators , otherwise you are going to make any problem worse than what it is.If your boiler starts banging within 20 secs of turning it on, it sounds like your pump has burnt out. Depending on what type of system you have got, and the age of your boiler, its a bit difficult to say exactly whats wrong. If you feel the pump after its been on 10mins and its red hot , thats where your problem lies, but i would get a decent gas safe enginer to come and check your system/boiler.