You said "under" the car. Both my and my cousins' mustangs as well as some other cars I've heard make this sound like marbles upon braking or acceleration. I think it comes from the exhaust. I thought that maybe a piece of weld came loose inside the piping or a piece of catalytic element broke off and was rattling around in there. If thats what your talking about then I wouldn't worry about it cause there's nothing you can do. Just a suggestion.
------------------
Blueneonman -
'95 5.0L Mustang GT Convertible | K&N Custom Cold Air Intake | FMS 9mm Wires | Hollow Cats | Flowmaster 2 Chambers | BBK Subframes | 6-pt Rollcage | King Cobra Clutch | FMS Adjustable Clutch Cable | B&M Short Throw | Coolest Neon Lighting on a Mustang EVER!
Homepage:
http://www16.brinkster.com/blueneonman/