Its a common thing for thrust bearings to rattle, because they are being pushed into the diaphram springs. Resting your foot on the pedal is enough to touch the bearing on the springs and stop it rattling (this also being the reason u shouldnt drive long distances resting your foot on the clutch pedal )

As long as it is the thrust bearing, which it sounds like it is, then it should be fine. If its literally brand new give it a few miles and it may stop anyway.

I don't know if the pedals on sierras are adjustable or self adjusting, most fords are self adjusting. But even if you could adjust it you wouldn't want to because it would have the same effect as resting your foot on the pedal.....wearing everything out quicker.