Join Date: Mar 2011
Location: South Western Ontario
If you're burning propane (LP) it could just be an extinction pop when the flame shuts off. Propane is heavier than air, so when the burner is shut off, it will burn down to the height of the burner, and then suddenly ignite all the gas sitting in the burner, making quite a pop. If you're burning Natural Gas (NG) I would suggest a manometer test to make sure your valve is shutting off properly.
The valve works off a very fine electromagnetic coil that lifts and drops a plunger using only 750 mv. It could be that this plunger is stuck, seating crookedly, or that the coil is burned out. A new valve can be pricey though, so hold on to your hat, it could be as much as $450 to get a new valve installed. Either way, I'd suggest getting a fireplace expert to look at it, it'll likely cost you $90, but you will know for sure what the matter is.