Strange noise coming from furnace after AC has shut off. Normal?

I just moved into a new place and it's the first I've ever had central air. The furnace is a Heil single stage unit. When the AC has been running and turns off, either manually or automatically by the thermostat, the blower shuts off and everything seems fine. However, then about an hour or after it's been off, the furnace makes a loud 'whir' sound that lasts two or three seconds and then cuts off. Sometimes it'll happen one or two times after the first time, about 5 minutes apart. Again, this only seems like when the AC has been running. It seems like a distinct 'whir' sound and not like when the blower turns on.
Is this normal? I'm not used to a central AC system. If it's normal, what's it doing? Thanks.
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snipped-for-privacy@hotmail.com wrote:

It might be the condensate pump. Look around the outside of the furnace, probably near the floor for a small square-ish looking pump. There should be a drain hose or tube leading down from the AC coil to this, then another tube leading from this to a drain or sump nearby. They're usually float-controlled, so you're hearing it pump as its reservoir fills up.
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Are condensate pumps the norm? Never seen one before in a few systems I have looked at.
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snipped-for-privacy@gmail.com wrote:

Mine doesn't have one either - just gravity flow from the pan to the drain. However if the drain was higher than the pan I would see a need for one.
Bob
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Bob wrote:

Just checked out a system. There is a small trap in the tubing just outside of the coil unit. Then slow gravity slope down to the drain. Anyone else have a trap?
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snipped-for-privacy@gmail.com wrote:

yep
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snipped-for-privacy@gmail.com wrote:

Not all that unusual, but if it is possible to avoid using one I highly recommend it.
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Joseph Meehan

Dia duit
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louie wrote:

I'll check it out. But does condenstation continue to form even after the furnace and the condensor have shut off?
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snipped-for-privacy@hotmail.com wrote:

Ack, I meant the furnace and the compressor.
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snipped-for-privacy@hotmail.com wrote:

Not to steal fuzzies thread, but here's a related question: Is it normal for the outside condenser fan to run a couple of minutes after the inside unit has shut down?
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