It's the humidifier. I can see it from here.
"Mikepier" wrote in message
oups.com...

keni wrote:
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

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.