Water Heater Vent Flame
When my gas water heater fires up, there's sometimes a flash of flame at the vent opening, where the vent pipe attaches and a sharp whop sound.
It's a GE model SG40T12TXK00 - a 40 gal, Ultra Low-NOx unit, less than a year old. I think this just started to happen. I keep the intake vents clean, so I don't think that's the cause.