I have a two-stage Atwood furnace, the model 2334. It operates at 23k btus or 34k btus and has a two speed fan as well. It is very quiet in normal operation, i.e. 23k btu and low fan speed. I've heard the fan speed kick into high when the set temp on the thermostat is several degrees above the actual inside temp, but not sure if that means the burner also went into 34k btu mode. The high speed mode is definitely noisier, though not all that much.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL