When our lennox furnace first starts up it makes a banging/knocking noise anywhere from 1-5 popping/knocking/banging. This doesn't always happen but quite often. Any suggestions? We were told it could be the pipes up in the attice expanding but we have thoroughly wrapped/insulated them.

Is the noise right when the burners light? Or is right when the blower starts? If it's a 90% furnace you may hear expansion from the pvc pipe if it was not hung and supported right or pipe pinetration may be to tight. If its after the blower starts it's a air flow problem--check all return air filters.