Start dismantling. Heard of a 2014 MDX with a bolt rolling around in the second row vent. If you can get a hold of a bore scope or one of the newer inspection cameras with a long lead you may be able to sneak it down into the vent and have a look. This one was a Snap-on camera with a magnet on the end that was able to fish the bolt out of the vent without removing anything. The RL shift interlock noise is not even close to being as loud as the TL and TSX. If that is the worst you gotta worry about, then that is pretty damn good. If your shift interlock solenoid went bad on a Cadillac CTS or STS of the same year, you would have to replace the whole shifter assembly. Good luck.

 

It is likely the shifter lockout solenoid mentioned above.

Mine made that sound and it was replaced under warranty a few years ago. My tech told me it would return with the new one as most Acura's exhibit this sound. The RL is just quieter inside that you hear sounds normally muffled by normal operating sounds.

Sure enough it started again last year. It does sound like a marble shifting about, but is simply the solenoid piston engaging and disengaging after every stop, then start.