Mid efficiency as in 80%? If you retreive the model number (and brand) from the data plate (possibly a sticker) from the furnace then we can assist you much better than only assuming.I'm pretty sure that it's mid efficiency.
It was installed 11 years ago when the house was built. I'm almost positive it's an 80%. Why would it just rattle when the burners are on and the blower is off. Could it be a flapper in the flue? If it is, how is the best way to check? On the roof?