Noise when furnace kicks on?

Question

 We have a gas furnace and im not sure how else to explain this... When it goes to kick on i hear this loud noise. Sort of sounds like a metal detector would over a certain type of metal. Or maybe robotic or alien like if you ask the children here lol. It lasts for maybe 4 to 5 seconds as its kicking on then stops. Everything else sounds normal. Its working. But i don't know anything about them. I've always had electric heat. Could it be the gas line? I dont smell a leak. Please help. 

Answer 1

That sounds like a spark ignition system.

Some furnaces ignite with a glowing hot heat element, others with a pilot flame (uncommon in modern applications) and some with a spark ignitor. The high voltage spark sometimes makes a noise you can hear.

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Answer 2

Fire Box heating up makes spooky noises.

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Answer 3

Something in there is maybe loose, misfit, or rattling. Don't mess with it yourself. Get someone with experience.

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Answer 4

No, the metal ducts (and furnace) are expanding with the heat.

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Answer 5

good to know i guess

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