ugh.. just when ya think everythings fixed, something else pops up..

Okay, my 2008 FX4 only has 4000 miles on it and the Reverse seems likes its acting up when stopped. It runs fine in Drive, Park, and Neutral.. but when it's in reverse and I brake, the truck feels thirsty.. The RPMs don't fluctuate, they're right at about 750, but there is a definate contstant shudder/rumble in the brake pedal when fully stopped. It kinda makes a dry scratchy sound, and it goes away when I give it gas and I'm actually moving..

Any ideas? I've only recently noticed it making this noise within the past 2 weeks, thinking maybe it would go away, but it hasn't. It doesn't really bother me too much, in that I don't often drive in reverse.. but I'd rather make sure this isn't a more serious underlying issue being that my truck is so new.