My heat n glo makes a loud bang when you turn it on, (several seconds after flipping the switch). The flame comes on right away and then a few seconds later... BANG. It never did this before and if it did, it was not this loud because I would have noticed it. It doesn't make the noise when you turn it off, only when you turn it on. What could this be, is there anything I can do myself to fix it or do I need to call in a repair man? Thanks