New Carrier 59TP6 gas furnace whistling sound when burners on

I bought a new Carrier 59TP6 gas furnace last year, and ever since it was installed, there's been this whistling noise when the burners are on. My old Heil furnace did not have this problem but it was old and broke so I replaced it.

The whistling sound starts when the blue flames ignite. When the blue flame disappears, the whistling immediately stops. There is no sound when only the blower is on. The whistling is audible when standing beside the furnace, and it gets louder as it travels through the ductwork. In the bedrooms, the whistle becomes loud enough to wake me up every night.

I've had technicians come twice, and both times they said everything looks fine and they couldn't tell what was causing the noise.

The sound seems to originate near the top of the furnace. Like where the furnace connects to the ductwork above.

Other info: I have a 5/8" flex pipe leading into my furnace. Ductwork did not change when the new furnace was installed.

Any guesses about what might be causing the whistle? Could the sound be coming from the heat exchanger?

I can get the tech to come out a 3rd time but I think they'll just say everything is fine again and leave.

source https://hvac-talk.com/vbb/threads/2227317-New-Carrier-59TP6-gas-furnace-whistling-sound-when-burners-on?goto=newpost