furnace pre-ignite - jet engine

Was over at my wife's sister's house... She has a ranch style house, with the furnace in a "closet" in the middle of the first floor.
When it first starts up for heating, it sounds like a jet engine winding up... Later you hear the burners kick on, and then later the main blower.
What the heck is the pre-ignition sound ?
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On 12/7/2013 12:11 AM, ps56k wrote:

Probably an "inducer fan". Blows air into the burner box, and blows the flue gasses out the chimney. Earlier furnaces used the heat of the flue gas to carry the gas up the chimney. This lost a lot of heat into the sky. Newer furnace use a fan to blow the gasses out.
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