I have a 97 2 door Tahoe. It has the 5.7 vortec. It was running great. Just redid the injection system. It was running amazing. Drove to work. Worked all night. I went out in the morning and warmed it up it was fine. As soon as I left the parking lot there is am extremely loud knock that seems like it' coming from the back side or underneath the motor. I have a feeling it' my flywheel but I'm not sure. I tried to get the inspection plate off but my exhaust runs right under it so.i have to break it loose and drop it before I can inpect it. It doesn't have any loss in power. Oil pressure is normal. Other than the noise it runs and drives as it did. I have a video of it idling but I don't know how to post it on here.

 

I have a 2005 Impala with the 3.8 engine. It would make a noise while driving that sounded like a collapsed lifter until you lifted the hood and listened a little closer. The noise was coming from deeper in the engine bay, turned out to be a cracked flex plate / flywheel. I know for a while this was a common problem with GM but don't know if it extended to your truck. Might want to put the truck up on jack stands and carefully start the engine while you are underneath it and see if the noise is coming from inside the inspection cover for the flywheel/torque converter.