No idea really. I just meant to go push/pull on things to see if anything is loose or if you hear a similar sound while doing so, it's probably a long shot but that's what I'd do.

Obviously it's hard to say since I haven't heard the sound, but there's been times when I thought "wtf was that" at a sound before realizing it was something in my glove box or, this wouldn't apply to your situation, something in the trunk.
Just as an example - Occasionally my car makes a "tick" sound, hard to describe, when I'm pulling into a parking space. Has to be a bump somewhere as I'm approaching it. Sounds as if something might be loose by the glove box, but it's actually the ABS reacting as the inner wheel locks for a millisecond as it comes off the bump.