If the "Check Engine" light is on, have someone get the OBDII codes with an OBDII scan tool. If not, check the plugs, distributor, plug wires, timing chain, and compression.
SOURCE: 2001 Jeep grand cherokee motor knock 4.7 motor
First, if the noise is from the upper part of the engine and is more of a tapping than a knock, likely you have a problem in the valvetrain, most often a bad lifter or worn rocker arm. If the noise is low, deep inside, the noise is from a bearing. This is generally accompanied by lower than normal oil pressure. Regardless of the underlying cause, correcting the cause will not stop the knock. To do that correctly, the engine needs to be removed and all crankshaft bearings and the crankshaft itself need to be replaced (crank can sometimes be "undercut" and thicker bearing shells can be used depending upon the amount of wear.) If caught very early, sometimes only one bearing can be replaced in-car, but in 99% of all cases I have ever seen, most will fail again, sometimes more severely if done this way.
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