I have a 1995 Buick Regal. It makes a popping noise that is coming from the transmission when it is in park but not when it is in gear. It shifts through the gears fine while driving. It only makes the sound sometimes. It seems to be as soon as the vehicle starts..then it goes away but if you push on the throttle it starts again. Also, if you put your hand on the transmission casing under the car you can feel the pops. Any ideas what this could be?