I've noticed that after turning off my TH Exec, a faint (even with the doors open) ratching creak can be heard from the engine bay. It sounds similar to someone pulling the hand brake on from 0 to full over the course of a second (not quickly but not dead slow).

I'm not sure if it happens if I leave it in Drive or Neutral as I always switch it off when it's in Park.

The noise isn't loud or dangerous sounding. It can be heard the second the motor falls silent.

See if you can get someone to turn it off while you have your head under the hood and hopefully get an idea of where abouts it comes from. Also maybe try in Neutral.

It's the Idle Speed Controller (ISC) returning to its default position. My TL does it too.