If you haven't read up on it PLEASE remind them to check the oil regularly. At every fill up, and maybe more often until you know how much oil your particular VX uses (if any at all). The engines are prone to drinking oil. Some of them sip oil while others guzzle it. Just don't want the engine to go kablooey on them on the way home.
OK, back to your issue; did your son say where the rattling was coming from? The heat shields are a likely suspect, but it could be something else. Since it goes away when in park it more than likely isn't coming from the engine. There have been reports of ticking sounds coming from the engines when/if the timing belt tension pulley goes bad.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless