noise from fan on isar 30 he ?

isar 30 he boiler when you turn the heating on the boiler flares up and the central heating starts to run constantly for about 4 to 5 mins after about 5 minutes the boiler starts banging and rumbling .i have traced this to the fan area also when i have looked at the flue out side when the gas injects into the boiler there is a loud rumble noise any one have any idea of the problem

john