I have a 2000 Coachmen Catalina motorhome (Class A diesel pusher) with a 4-spd Allison transmission. I notice a slight noise - almost a deep hum, like going over those grated asphalt patches on the shoulder that are meant to keep you awake - when I'm going downhill; is this the transmission?

T.D.

You're positive that it's just a 4 speed?

Could be driveline slack that only groans on the coast side of the ring gear etc.. Could be something entirely different also. Need more information to help you out.