The mechanical sound is what makes this hard really help much... Usually, a mechanical sound comes along with a mechanical problem
! (Hope I'm wrong.).
You might try disconnecting the Chassis Battery cable at the terminal for about 15 minutes, and see if that resets the Allison ECM.
And I'd suggest you ask this to be moved to the Allison part of the forum, under Powertrain Garage... As it should not be unique to a CC.
Best of luck, and please let us know what you find out,