freeing the gear at red lights(auto)

[Hocky]

I am in the habit of freeing the gears when my car is at the red lights, more often when its waiting for the surburban trains to cross left to right and right to left(this takes longer if more trains come along at peak hour). If the road is flat, I just free the gears and there is a slight clunk noise and there is a bump like something hit from behind felt. Is this a normal feeling when you change from drive to N or P at standstill? If not normal what can be wrong to cause this? If I carry on this thing will it hurt my trans or engine or drivetrain? My Camry has no clunk or feel of any jerk when this is done. The stationary gearchange is smooth with no jerk at standstill. It feels a lot better. Anybody?