I just picked up a 2001 ram 1500 with automatic transmission and the 5.9. I went to pick up my daughter from work yesterday afternoon. It was about 90 degrees, so I sat in the truck with engine running for about 10 minutes. After we left her work, we drove over to McDonald's. I left the truck running, again for about 5 minutes while i went to grab the food. When I came back to the truck, I could hear a knocking sound. There was no check engine light or overheating issues. Under load, I could still hear the knock. Drove it home and let it sit overnight. Checked the oil and it is clean and full. Restarted the truck and the knock is gone. Any ideas as to what or where I should start.

 

First, determine what the noise really is. Might be anything from an exhaust leak, to a bad motor mount, or a host of other things. Also, what is oil pressure like when the engine is fully warmed up.

 

Hearing the knocking noise only during heavy loads of use and with high temperatures, knocking is the piston hitting the cylinder wall. Here is the reason why. Due the engine having high mileage, the heat over the lifetime of the engine, cause it to warp. Any metal material, exposed to high levels of heat, for an extended period of time, will cause different surface contact. Heavy loads is where the engine is at its hottest. If you want to verify, check the mileage and when the vehicle was built. Unfortunately, there is not many options, replace the engine or replace the vehicle.

 

I assume the ac was on, Did you shut it off to see if that was the cause? Also what weight oil do you have in it?