Allroad

A6 Allroad 2001 auto transmission. I get a click or snap noise when I put it in reverse or forward while on a hill. This happens when I park and release the brake to get out of the car, or when I get in, start up, and put it in gear. Doesn't happen when I'm on level ground. Any ideas on what that click or snapping sound is? It's not a "thunk" kind of noise when you put it in gear, it's a clear, crisp snap or click sound, definitely metalic sounding. I should add that there is one click or snap, not a series of clicks or snaps.

If I set the emergency brake before I release the foot brake it doesn't happen. So it must be some kind of movement after releasing the foot brake thereby putting the pressure on the transmission which is now in park position - the weight of the car and gravity pulling on the transmission as the car wants to go downhill about 1/2 an inch.