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 *