My son went to a friends house and was on a hill he placed it in park and it kept moving and he said he heard a clicking noise from under neath. I verified that I can hear it clicking whilein park coming from the transmission case (inside) any ideas??  

the park pawl is held in place by a spring loaded rod, It may be bent or the pawl could be worn, worst case is some of the cast cogs on the planetaries may broken or worn, the cause is from frequently putting the tranny in park before its completely stopped.

if it has the th400 the guide plate bolts are loose or the rod is most likely the problem, it can be easily accesed by removing the pan.

on the torqueflite, the rod is accesible from the removing the pan, but the pawl is in the t-case adapter the gear is under the governor against the case.  

Thanks a ton great info