What is making this buzzing sound (gas furnace)?

yuri said:

It sounds like the gas valve is buzzing.

That is a 30+ yr old very old furnace and you may want to replace it B4 the heat exchanger cracks.

The gas valve may fail soon and stay stuck closed.

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The gas valve was replaced in 2016.
I figured out the problem -- I got an upgraded WiFi thermostat that claimed it did not require a "C" wire for power.
It worked on the batteries just fine, but every now and then, it would try to pull power from the furnace and it caused that awful buzzing sound.
I got a "C" wire power adapter and it fixed the problem.

What would cause a heat exchanger to crack? The furnace is probably original to the house which is over 50 years old. I'm actually impressed with how well the furnace works and my gas bill isn't much during the winter at all. I see ads guaranteeing utility bill savings with more efficient units, but it would take 2 lifetimes of one of those newer, unreliable furnaces to see that savings.

They crack from thermal expansion and contraction/bending from heating and cooling and if you look closely at some the metal turns purple and gets caustic embrittlement. There is sulphur and moisture in the smoke and eventually the metal gets brittle/corroded and breaks.

I have seen them burst like a glass windshield being hit by a rock when they get over 25 yrs old.

Like a pop can if you squash it and bend it back and forth a few dozen times it will crack.