There are only 2 things that come to mind. One it could be the shift interlock your hearing. This is the solenoid that allows the vehicle shifter to lock, you must step on the brake to release it. Second thing I was thinking it might be would be the parking pawl. This is the lever in the transmission that locks the vehicle in park. Did you notice if it only happens on sloped ground? Do you feel the vehicle roll slightly the hear the click?