Greetings,

I have a Lennox 80MGF furnace that is making loud noises from time to
time.
I first thought that the blower's ball bearings or something was bad
and that the motor action is what was producing the loud noises.

The noise happens only sometimes and is rather loud.
After standing next to the furnace long enough for the sound to appear
again, as it's intermittent, I now think that the noise is coming from
a small box near the gas exhaust pipe near the wall where the pipe
exits through the wall from the basement to outside.

This little box looks has a flexible metal hose leading to the
furnace. It also has a solid hard small pipe line leading to the top
of the exhaust. I can't see where it exactly leads because it kind of
dissappears into the wall. The box is pretty much square and has a
small round thing on the side.

My ignorant mind is making the wild guess that this box takes samples
from the exhaust air, and determines that it does not like what it
smells, and is sounding an alarm.

I know I know, call in the gas guy. But I'd like to know a few things
before the gas guy starts telling me things.

If really needed, I could upload some digital pictures somewhere...