I have a natural gas furnace that was installed in 1991. During the last couple of days I've heard random noises (bangs) radiating up the vents. The bangs don't happen very often, every couple of hours or so, and while not deafening, are fairly loud. I'm not sure if this is from the vents popping due to the cold weather, its been in the 20s for a couple of weeks, or if its some sort of major warning sign that the furnace is in trouble. Any ideas?