I have a 2001 tahoe with a 5.3 vortec. When I first start it up, it has a noticeable knock coming from the crankcase. It sounds like a few rods are knocking. I have a hard time believing that it is a bad bearing or wear and tear. I've owned it since new and have anal with its maintenence and operation. It has 94,000 miles on it. Any ideas? thanks