Instantaneous water heater noise

So I was just in my backyard, and I heard a rather loud noise coming from my neighbor’s water heater flue. The noise sounded like a car with a loud muffler…it lasted about 20-30 secs and ended with a poof of steam. He had just had the unit installed In January. Anyone have any experience with an issue like this or what may cause it?

Clearing out a build up of condensation on start up?..guess.