What would make a rattling noise on startup in park or neutral?

What is your question?

When I start my truck in the morning after a short while a rattling noise can be heard in the rear of the engine where it mates to the transmission. The sound changes when I put the vehicle in drive or reverse (it gets quieter) I tried to find where it was coming from but could not pinpoint location. The Truck is still driving with no problems other than this noise that is making me nervous. Help!