DIAGNOSIS: That's a small gas explosion happening in the firebox. It's caused by gas accumulating in the burner tubes or firebox before ignition, or lingering after shutoff, and then igniting all at once.
FIX: Always open the lid before turning on the gas! Let any accumulated gas dissipate. When lighting, turn the knob to START/IGNITE immediately—don't pause with gas flowing but no spark. When shutting down, turn the knob fully off, then close the propane tank valve at the tank itself to burn off the gas in the line.
IF THAT DIDN'T WORK: You might have a small leak or a slow gas flow issue causing excess accumulation. Check your hose connections with a soapy water solution for bubbles.
HEADS UP: While startling, occasional small pops aren't usually dangerous on these grills, but consistent loud bangs can stress the metal. The lid-open habit is the single best fix.