I was in a hurry driving back to school on the interstate in my 99 Silverado 5.3L and noticed a loud whining knock coming from the engine bay with a drop of oil pressure down below 20. I was running about 85-88 for a while, then when I turned down my radio I noticed the knocking noise that sounded like I was driving a diesel. No check engine lights or any warning came on. I took and exit, parked and then it really sounded like I was driving a diesel. Gassed up, drove the rest of the way home and parked the truck. The next day, started it up and it was still whining like crazy make that same noise. Drove it in town and after a few minutes it stopped and oil pressure returned. Any Ideas?