knocking sound in engine

96 jeep cherokee. engine knocks when starting and when running but rides good. i don't know how serious it is but its sounds really bad. any suggestions on what the problem may be?

6 cyl, right?
Is it a knocking noise and not a valvetrain 'tapping' noise?
These Jeep engines are notorious for developing a piston slap, yet they will run very reliably for years while still making a noise. The piston slap is most noticable on cold start up and deminishes a bit.
Later Jeeps had a redesigned piston which reduces this problem.
Otherwise, it could be a rod (bad bearing) noise.
These noises are much more noticable at idle and under load. Engines don't last very long if a bearing is bad, and an engine rebuild is required.
Sometimes a worn oil pump can allow for a bad bearing noise only at idle when the engine is hot.
Thicker oil can help temporarily, but a new oil pump is usually the solution here.