Make sure it's not the transmission because a torque converter can make sounds like a tapping valvetrain. Where the engine and tranny meet up under the truck, there is a large cover that exposes the flywheel and torque converter. Unbolt the torque converter, spinning the engine clockwise on the front crankshaft bolt to turn the flywheel allowing you to get to all the bolts. Push the torque converter away from the flywheel then start the motor. That will separate any chance of it being a transmission noise over an engine noise. My noise was really loud and obnoxious. I thought I had a bad motor, turns out all I needed to do was replace the torque converter.