Heater Start up Backfire / Pop sound

[DWHeggen]

I have a Pentair Max E Flow gas heater. This problem started in the last 2 weeks. When the heater starts up everything seems fine until the burner ignition. When it starts I get a loud ban Backfire Pop sound like a muffled misfire. The burner starts but I know something is not right . Any help is appreciated. I can troubleshoot and do my own repairs just trying to fix myself to eliminate a costly service call.