I have hot water heat in my home, with a new boiler but quite old pipes. At night we hear a tapping or small knocking noise in some of the pipes. The pipes in question are not the copper ones that run through the floor board radiators, but larger cast iron pipes complete the circuit to the boiler. I know it is not the banging that comes from too much air in the system, because we've had that problem. This tapping is repetitive, there's almost a beat to it at times. From reading other posts I'm now thinking this may be from the expansion of pipes in areas that are too tight. My question is: Is this dangerous or troublesome for the system - or just a nuisance? Is it common? The pipes are in question are in an outer wall and would require some serious wall demolition to get to.

Thanks.