gas furnace burns at orifice

[cm126]

I have a Tempstar upflow condensing propane furnace. It will light normally, but sometimes during a run cycle, I will hear a pop followed by a wavering/whistling/vibrating/fluttering...very hard to describe, sound. The sound is caused by the gas burning at the orifice with the flame blowing into the manifold tube. The pop at the begining of this is when the orifice lights. The flame at the burner is only slightly smaller than it is normally. Usually, it will correct itself after a period of time. If I blow a gentle puff of air into the air intake of that burner, it returns to normal. This happens mostly to one burner. I have had all 3 burners replaced by local service guy. There is 10" W.C. pressure at the burner valve test port as specified in the manual.

What is going on and how do I fix *

Thank You
Craig

[nomadpeo]

maybe your regulator is starting to flake out.

[cm126]

I can see the regulator dropping the pressure as a cause, so I tried to test that theory. I killed some time in front of the furnace today, watching the manometer connected to the valve and waiting for a flashback. Took a while, but when it happened, the pressure didn't even twitch. I rechecked the pressure at that point. It was at 10.5" Just for kicks, I turned the gas valve regulator down, hoping to see what it took to make it flashback. I got down to 6.5 with no problem and quit at that point. It ran at 6.5" for maybe 30 seconds before I put it back at 10.5. I doubt that a momentary pressure drop of 4"+ would go unregistered by the manometer.

And how would the thermocouple cause flashback.

Craig