When I start my jeep (1997 Jeep Grand Cherokee loredo 4.0 inline 6) it has a weird knocking sound it’s louder when it’s first started after it’s cooled down but if I hit 3,500-4,000 RPMs you can hear it slightly but I never hear it going down the road. What can this be? Someone please help.

 

I had the exact same problem and assumed it was piston slap, but in teardown several years later, I found #5 connecting rod bearing was worn down to the copper! It never showed a reduction in oil pressure either.