Explosive booming sounds coming from my water heater

As per the video, recently I’ve been getting these booming flashes of flame coming from the bottom of my State Select 75gal power vent water heater.

Unit’s about 9 years old. No leaks. Hot water throughout the house just fine. But these sounds are rattling the entire house and, quite frankly, scaring us.

Anybody have any ideas on what it could be, and what the fix is? Many thanks in advance.