Have you found out what the problem is?

 

Mine does that to it is related to the pcv valve on the passenger valve cover. Pull out the pcv and shut off the motor and all you will hear is a hissing until all vacuum is gone but no rattling. I believe the little ball bearing in the pcv makes the noise and the fact that it is on the valve cover amplifies the sound. I put in a new one and the sound went away but it came back later....Not an issue just annoying!