I have an older mobile home with a Coleman gas furnace. It has recently started making a single, loud bang before it starts blowing air. Sounds kind of like a car door slamming. Tried cleaning it and it has a new filter, which made the banging a little less frequent, but now it's back to doing it almost every time the heat kicks on. Any ides? I tried watching it to see if I could see something wrong, but when I have the door open it doesn't do it.

It could be something to do with the blower itself. Does the gas burner itself come on with no problems?

There are 3 rows... 2 light fine and quickly, while the 3rd is about 5-10 seconds slower to light and sometimes has a small flash flame at one end.

I was afraid of that. With such a long delay it is possible that

The burner flame over areas are bad
The gas pressure needs to be adjusted.
The orifices are restricted.
The heat exchanger is bad

Please have a professional Mobile Home HVAC contractor service your unit right away. The condition you have now is very dangerous and a homeowner has no business trying to repair it. Sorry but that is the way I see this one.

OK, thank you very much for the help. Looks like it's time to call the ex LOL he's an HVAC contractor.