At approximately 2000 RPM's many aftermarket cat back exhaust systems will have a definite hum or drone in the exhaust note. This can get very annoying when driving for extended periods at highway speeds where the RPM's stay right at this "sweet spot". This annoying tone that drills into your brain after a while is known as then exhaust drone.
drone - 1. to make a dull, continued, low, monotonous sound; hum; buzz.
U.M.