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