Well, after the night when I first heard the noise, it hasn't been heard since. So I've cancelled the dealer appointment. It would be hard for them to troubleshoot something that is not re-produceable at this time.
The Xterra is due for it's initial regular service soon; I will bring it up then. If it turns out to be anything out-of-the-ordinary, I will post it here for others to reference.
Thanks all!