Originally Posted by GorillaRE

Have you ever really listened to a loud un-muffled rotary idle? Do you know how the idle seems to be fairly smooth/consistent and then all of a sudden not? e.g. brrrrrrrrrruh........brrrrrrrrrrrrruh........brrrr rrrrrrrruh.........breeehhhrrruuh etc.

That is the sound of too much exhaust pollution. It builds up and builds up and builds up, then breeehhhrrruuh.