I have a 2001 Chrysler LHS with 3.5 litre engine. Suddenly a knocking sound started and is loud. Sounds like a clanking noise. A repairman said it is

SOURCE: MY ENGINE KNOCKS AFTER DRIVING A FEW MILES

you have a rod knocking in the engine

SOURCE: Knocking Noise

you have a rod going out on the car

SOURCE: Real loud knocking noise when starting

this can be a serious problem, sounds like one of your engine valves, has the car overheated recently? if your valves bent, you need to take the engine apart hope i helped

SOURCE: Loud engine knocking on 2001 Ford Focus

Sounds to me like a very common problem has occured to your neighbor. If this is a SOHC engine, one of the valve seats in the number 4 cylinder has fallen out. I'm sure it has damaged the head and the piston. Stop trying to run it or you might blow a hole in the cylinder wall or break a rod or both.

SOURCE: My 2000 1.6 litre Corolla has a loud knocking from

STOP RUNNING THE MOTOR IMMEDIATELY!!! If it has dropped a valve all you are doing is making the damage worse. Take the valve cover off and make sure all of the valve stems are there. If not replace or rebuild the motor.