Join Date: Sep 2005
Location: San Tan Valley Arizona
Some rocker arms WILL be loose when the engine is off. When there is no oil pressure, the hydraulic valve lifters will leak down, and result in some valve lash when the valve is in the closed position.
The knocking at idle can be a valve train noise or piston slap. The valve noise is more metallic. A dowel rod or stethoscope will help isolate which is which. If they are minor, neither noise is an indication of a serious issue.
Rocker arm bridges or associated pieces normally only need replacment when they show signs of wear due to dirt in the oil or oil starvation. The amount of gunk in the rocker cover area can be an indicator of wear or starvation, but rocker removal and inspection is the best way.
I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
So...I stick around and give advice.