Combustion noise

We recently had a Dutch west vented gas (propane) cast iron stove installed. The installers had to come back a few days later as initially they had the wrong collar, and when they did, they noticed the glass was very dirty. We hadn't noticed because the screen hides it. They said the cause was supposedly the log set was off a little and they adjusted it a bit. Since that time, we hear a slight fluttering sound from the flames, which we didn't hear before. It seems more pronounced on the lowest flame setting. Once the fan comes on of course you can't hear it.
They don't seem too concerned about it, but is this normal? It seems like it was silent before, but we only used it for a couple of days. Other than that, it's working fine. Thanks for your thoughts. Tony

Let me rephrase my question: those of you with direct vent gas stoves, is there any audible sound when the flames are burning, or is it completely silent?

I'll try to post something later, but I have the stove off right now, as they're coming to look at it later. But let me go over what's happened so far: they installed stove and it ran quietly and with the correct flame pattern as shown in manual. They came back less than a week later with an unrelated part to install, (wrong collar came with stove) and when they had it apart, noticed the glass was very dirty. They said the log set needed adjusting, which they did. The next day I noticed the fluttering sound, and also you could hear the gas flow slightly. I called them and they are coming out later. But yesterday, I took the glass front off to check it, and it was very dirty again. I cleaned with the cleaner and reinstalled it. Today it's dirty again! I'm starting to think the log set was not the cause of the sooting in the first place, but I don't know what is. All I know for sure is it was quiet before, and it's still getting dirty. Also, the flame pattern is not like the picture anymore, as there are no longer two smaller flames front of log set, just the three large ones in back. Any thoughts? Thanks a lot.......tony

I'm pretty sure it's fixed now. The service guy just left, (seemed far more knowledgeable than the installers), and he did four things: 1.opened air shutter completely.....it was mostly open but not all the way 2.spoke with tech from my propane company and raised pressure from 10 lbs to 12lbs. 3. Adjusted log set which had a lot of soot on them from flame hitting them and 4.replaced the solid embers with the ones that look like steel wool. Quite happy with it now: good flame pattern and silent. Thanks for your help and support. Happy new year. Tony