That I have personnal knowledge of makes this "wine glass" sound. The sound starts about 15 seconds after the burners first light. The sound lasts for about 30 seconds and then fades away. It has done this since new. The source seems to be from the burners which are shaped somewhat like a "horn"
A Lennox furnace rep once suggested I could try laying a steel rod across the top of the burners to dampen the sound. I never did, but it might just work.