Hi, sounds like the system needs a clean, Get a cleaner from a good diy store and put it in the header tank, if its a combi system, put it in a radiator and repressurise, Hope this helps
It probably isn't what the above postings suggest. The key is that you say it does it when the heating comes on. It is caused by what you suggested yourself - the pipes banging against the joists. As the water heats up the copper expands (it will only happen with copper - not plastic) and the pipes move slightly. Where they are too tight through a slot cut in the joist or noggin, they move in a series of 'jumps' that sound like bangs. I'm afraid it is very difficult to stop it.
You possibly have thermostatic valves on the return instead of the flow in some instances. When you hear the banging go round one at a time and turn them off, you will also feel vibration at the valve. At least you will know if it's not the valves. Hard@it
if its oil fired ,the water is static in the pipes when off. on startup the pump kicks in to assist water circulation therefore creating a momentary surge, any pipes not properly affixed especially long ones will vibrate on joists and other fixings, hope this helps