It's possible the rattle was from a loose nut screw, although that wouldn't explain the similar rattle I heard on the Schecter bass. I didn't strip the truss rod, as I said, both nuts turned freely & never bottomed out & I only had to turn them about a half turn to get the neck straight. Not sure how muting the A string will affect a rattle on the B string but after playing the bass for the 3rd time I didn't hear the rattle through my amp that I brought to test it with & felt like it was a non-issue at that point.  Even with dead strings it sounded full & punchy through my amp.

Here's what I found in the file:
 
top: California Walnut
accent: Maple pinstripe
body: Mahogany
neck: 5-piece Maple with 4 Walnut pinstripes
scale: 34 long
fingerboard: Ebony
side markers: bronze dots
pickups: MXY56
controls: Epic
finish: satin natural polyurethane (oil-look)
originally made for: Guitar Center, TX
birthday: September 8, 1998
 
Except for the fact that it wound up in a pawn shop, we haven't heard from this bass since it was shipped to Texas all those years ago.

Thanks Mica! This is the bass that got the ball rolling for me to eventually wind up with my Blue Orca! Before investigating this bass I had no desire for a 5 string, now my Europa is my main bass!