I for the life of me can't find the source of this rattling noise. I was holding the recorder near the front jack point on the driver's side. As you'll note from the file, the sound almost completely disappears when even the tiniest amount of throttle is applied. But comes back with avengence when at idle.
The noise is loudest on either side of the car near the front jackpoints. It is substantially quieter when you are standing by the front bumper.

http://home.comcast.net/~grindmarc/merc_rattle.MP3

Thanks in advance everyone that replies.
I rebuilt the steering pump over the weekend, so I'm ready to tackle this one!