I have a 2017 f-150 5.0 coyote that rattles when it cold starts but goes away once the truck is put into reverse. Sounds like exhaust or cadillac converter is jacked. Anyone have any experience with this?

 

Just to be clear this noise only occurs when the engine is cold on startup and the only way to make it go away is to shift to reverse?

Does it go away when you shift to reverse before moving backwards or only after moving backwards?
It will never go away if started cold and left to idle in park for a while or if started cold and put in drive or neutral?

 

if i leave the truck on for longer then 1 minute it typically goes away but if i put it in reverse it immediately goes away.

 

I double checked and there arent any TSB's for noise issues with the 5.0.
You can start the vehicle and check under it if the noise is drastically louder at the exhaust.
Generally noise from the exhaust is a loose heat shield usually with a broken weld, that can be easily fixed with band clamps though.
I personally have only seen one cat that the beehive portion was just a little loose and made a rattling noise when cold, my assumption in that case was when it got hot it expanded just enough to stop any movement.
Based on your year you are nearing the end of the base warranty period, now power train warranty is till 60K I believe and emissions warranty which I dont recall how long it goes for covers cats but not the entire exhaust which can be rather annoying at time.