only when it fires up, but not always instantaneously, sometimes you can hear it after the has started whirring away, about 5- 10 seconds later. Also you can hear it when the boiler fires up occasionally (I preume to keep the heat exchangeer hot enough?)
I've got a 2 year old ISAR HE 35 boiler which to be honest has been nothing but a problem since we've had it! The biggest problem we have at the moment is that when the hot water is run there is a large audible bang from the flue (louder outside than when inside house) much like a car back...