I have a 03 GMC Envoy with about 110,000 miles on it. I know the fan clutch is bad and its been ticking for a while. The motor just started knocking here lately. It will start up no problem and idle smooth as glass but it will knock and has no power. It almost seem like its holding its self back. The noise seems to be comeing from the topend. It was low on oil and I added to it and it seemed to quit it down some. The vehicle doesn't smoke or leak. Any Ideas? The check engine light was on but it went away when it started knocking.....go figure.