I don't know what the noise is, but be sure to check everywhere as noises and vibrations can really travel through our little car. When my drivers side inner CV race was going bad all of the vibrations and grinding that I heard/felt, seemed like they were coming from right under my feet. Because of that I thought it might have been transmission/transfer case related, but it ended up that the vibrations where traveling from the solid mounted center section of the front axle, through the frame, and I was feeling it right under my feet.
"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