That sounds like the lens has thrown a barring, I have a… a few… lens do that. If the body itself is making the noise, then you threw one of the barring or one of the gears inside the body.
I have had a D300 burst open on me, and trust me, you won’t be able to fix anything inside yourself, there are so many small parts inside, and crap like you have never seen!
If you dropped the camera on the lens first, then the body hit, you most likely bent the focus rod (that tinny little piece of metal that operates the lenses auto focus)
Regardless, I suggest taking it your local camera store or Nikon realer, they can give you an estimate.
In my experiences the most costly fix I have ever had was my 80–400mm lens, I dropped it, bending the shaft of the lens, cracking some glass and stuff, it cost me 500$ to fix. The exploded D300 only cost me 300$ to fix, (but the dumb A**es at Nikon didn’t fix it all the way, my viewfinder is still all junked up, but it’s my backup camera, so it doesn’t effect me that much.)
I would say it shouldn’t cost any more then 300$ to fix the thing.