My son's 2000 Nissan Xterra would not start. He tried until the battery was near dead. The next day I went to help and it started but ran rough. We drove it to an auto parts store to see what code was coming up with the service engine light on. They said it was the knock sensor. We got it home and I took the top of the engine apart to replace the sensor. After making the replacement of the sensor and putting it back together it started but runs rougher than before. I have cleaned the MAF sensor with the proper cleaner, removed the air filter and moved the district or clockwise to get the best idle. It still runs rough, fails when reved. There is a light clacking sound from the passenger side; lifters or exhaust perhaps. The distributor, wires and plugs are 8 months old. There is no check engine light on and there is little power if placed in gear. Help please?