Forced air system fan problem

Hope this isn't too dumb of a question. We have a forced air system and whenever it's on and hot air is blowing, a fan inside a container that's mounted on the side of a duct begins to spin. The problem is it's making a very loud kind of rattling noise as if something is loose. It's never sounded like that in the 7 years we've lived here until now. It stops spinning when the forced air stops, so it's not a problem of getting to shut on or off, just that I'm not sure why it's so loud now.
I feel silly as I'm not even sure exactly what this unit's function is (assuming it's for cooling?), but it seems to only go on when the thermostat is set a little high. It's also connected to a switch (set to on) that's mounted on the side of the furnace. This is a different switch from the main power for the furnace system. I don't know much about these systems, but I'm assuming this is different from the 'blower' in the furnace chamber.
Can someone please explain this to me? What could be wrong with the unit?
Thanks
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keni wrote:

I can think of three things it may be.
* Most current furnaces use a power vent to exhaust the combustion gases outside. If it is getting louder, it may need service soon. Have this one checked at once as it may leave you without heat if it goes out.
* It may be part of a humidifier. It may need service especially if it has not been serviced during the last 7 years.
* Some one may have added a booster fan to help get more air to a cold room. If it is getting louder then it needs to be serviced or removed.
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Joseph Meehan

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wrote:

Terrible, just terrible. Do you have your car serviced every 7 years too? Bubba
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This is Turtle.
As My Good Friend Jim Carry would say I have my furnace and car serviced every 20 years wiether it needs it or not.
TURTLE
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keni wrote:

It's sounds like it's the inducer fan, it's helps to blow the exhaust out the vent into the chimney. Like any motor, over time the bearings get worn and needs replacing. The inducer fan only comes on when the t-stat calls for heat.
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It's the humidifier. I can see it from here.

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This is Turtle.
Just a Minute here i can see the thing too but they need to move the Rolles Royce out of the carport to be sure and see it clearly but it could be a fanless humitifier.
TURTLE
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