While descending a steep hill recently, I shifted from Drive to the next lower (2d I'd guess) gear on the shiftor to use the transmission to aid in braking. The transmission gave off a sound as if the gears were not fully engaged...sort of a gear clash which wouldn't stop until I stopped the car, put it in park, and resumed underway in Drive. I have since noted that if I push the shifter forward slightly the noise stops (sometimes). Now I note that if I start off in the 2d gear position I more often than not experience the same sort of noise thing...which at any speed at all is changed into a buzzing sound that when I slow changes back to the original gear clash sound as speed decreases. This continues even if I shift back to drive. It doesn't seem to happen as often if I first use the selector button to shift out of OD before shifting to 2d gear, but that is also inconsistent. Funny thing is that the tranny works perfectly if I start out in Drive and don't manually down shift for any reason. Also no problem if I use the OD button to come out of 4th to 3rd gear. Any ideas as to the cause or corrective action I should take?