refrigerator "pop pop pop"

I recently purchased a used motorhome with a Norcold refrigerator model 1201LRIM. It has been modified under the recall, and does keep things cold.

Inside the coach, however, you can periodically hear "pop" or a series of several pops, like little backfires from the refrigerator when it's running on propane. It doesn't seem to be associated with startup or anything else. It's just random and can stop completely for hours at a time. One thing I do know is that it isn't the spark ignitor - which admittedly can sound about the same. The previous owner says this has been happening for a long time.

If you look at the burner from the outside, the "pops" are associated with a small and fleeting blue flame coming out the mixture holes. I took out the burner assembly and cleaned out the rust with compressed air, being super careful to not touch the orifice. I tried reducing the air intake by slightly covering some of the holes. I also tried reducing the propane flow with the manual cut-off valve next to the 12V solenoid. None of this had noticeable effect on the random backfires.

Anybody have any suggestions as to how to repair this? btw, Norcold says this is "not normal", but maybe that's the safe answer they give everyone!