My 97 does it too... If it only happens when its cold, try this: Look to see if you have an electric plug (regular house type w/ground) towards the front of the left frame rail (near radiator). If you do, this is a heater for your engine. Plug it in on those cold nights and when you start it in the morning - TA DAA - no knocking! PLUS your engine is already warm and ready to go!!! Try it and let me know if it works for you too....
