hotwater tank pops loudly

I have a 14 year old travel trailer with the original water heater and sometimes it makes loud popping noises after it's been running for a while and the flame gets irregular. It doesn't happen when lighting, it happens after it's been running a bit. It's enough to keep me from using the thing - I've done all my showering in the facilities of the campground I'm staying at so far (and I've been a full-timer for over a year, so that's a lot of showers ;) but eventually I'd love to do some boondocking and I need a working water heater so I can shower to do that. Very little technical know-how here, so I'll have to take it in somewhere to get looked at eventually - I can only hope that when I do someone who knows what they're doing will look at it.