I have a 2010 4.6 4x4. When I put the truck in reverse and slightly step on the gas there is a rattling sound from underneath somewhere. Once I put in drive it doesn't rattle anymore. It doesn't do in park or in reverse with my foot on the brake either.
Thanks!

 

Could be heat shields. Mine would rattle at certain rpms and in reverse/ low speeds

 

@fride360