Furnace Problems Any idea's

[GypsyCliff]

Hello all,

About four days ago my furnace began acting up. After the fans turned for about 60-90 seconds, we heard a fairly loud bang, kind of like a small firecracker going off. I shut it down right away, and tried it again the next night and heard the same sound. It was pretty scary so we did not use it again for the rest of the trip.

Anybody have an idea of what the problem might be? I'm handy enough to fix it (I think), but not sure where to start. Any ideas? Thanks.