Jeep won't start

so, i went out to pick up dinner tonight, (pizza in case anyone is wondering) come back out, put the key in, turn it over, it fires and starts for about a second and dies!?!?!? turn the key again and it just cranks over but wont fire. i wait a few second and try again. nothin. i try a few more times and then it finally fires, revs a little but i hear a "knocking" sound that follows the RPM's. i quickly shut it off. it was also running really rough while making the knocking noise. my first thought was crank position sensor, but could that cause a knocking sound? any thoughts from you guys? its a 2006 XK 5.7L 95K mi