"Build up in the cross over from the prior year"? Build up of what, gas? No way! Build up of debris (spider webs, etc)? That's what he is supposed to clean out! No furnace should start with even a little bang and a big bang that shoots exploding gas out from the front of the furnace is very dangerous. The problem is that a lot of gas is being released from the gas valve before the igniter circuit actuates. A properly funtioning furnace should allow only a limited, controlled amount of gas to be released from the PICK gas valve before the igniter circuit. I suspect that your furnace is improperly wired to allow the MAIN gas valve to operate before the igniter circuit actuates, thus releasing much, much too much gas and causing your big bang. RUN, not walk, to a competent service man. Also, call the company from which you buy gas. They should have someone out to your house within a couple hours. If they don't, call the Utility Regulatory Commission in your State and inform them about whats going on.