2006 Mazda 3 when in park car rolls

I would like to know what would make a car not stay in park and roll. The e brake works fine on the car. However , when the car is even on level fround and I put the car in park the car rolls forward , pretty fast. It even makes a clicking sound if its on a slight high and its in park....the car will roll back. Does this mean the transmission might be going out in the car. other than the parking probelm the car seems to shift normal.