Scroll down, see the "similar threads"? Start reading there. So many things it could be.
After that, isolate where the noise is coming from. With a buddy in the car and without you crawling under the car, have him put the car in N, listen for noise source under hood, transfer case, transmission, exhaust, etc. A long screwdriver or metal tubing can act as a stethescope. Repeat in drive with foot on the brake. Gotta figure out what's ticking before we can help.
