During a ride yesterday, i noticed my 05 Sp. 400 making some rattling noises during heavy braking. I was riding some bumpy terrain, at about 15-20 mph. When I applied the brakes, it sounded like loose suspension, but I rocked tires, etc., and didn't have any play.

 

Could be just normal driveline lash if the wheels were starting to lock up.

 

the back lash noise is more heard when the brakes are applied hard. its most likey the drive train noise your hearing.