Transmission problem

Not on my stang, but my dad's 98 F150. I don't know much about automatic transmissions, so I was hoping for some general advice.

In park, there is no noise. When it shifts to reverse or drive, there is a rattle from the transmission area. Almost sounds like a little piece of metal rattling around the bell housing.

When you shift from park to drive in an auto, what parts start moving? Torque converter? What would make noise even though the vehicle is still?

Thanks for any help.

I think there is over 150k on the truck, but I'm not positive.

When I was looking under the car to try and figure out where the noise was coming from, I was using a broomstick to try and feel parts for vibration. (very scientific) The trans didn't feel like it was vibrating or moving any unusual amount.

Would a failing trans mount produce a larger vibration?