SouthernOutlaw wrote:Have you checked the piston clearance. Piston skirt slap will sound like a problem wirh the crank shaft. Normally will start after a motor is warm to operating temp. I notice you dont hear it until after the motor has been running for 15 minutes or more.
This may be a stretch but what is the history of the pistons? I had a similar experience with a early 1990's GM 350 that had improperly sized aluminum pistons in a CI block from the factory. When the wear got to a certain point, the "slap" of the pistons in the bore was bad enough that it had to be rebuilt with new correctly sized pistons to stop the noise. In fact, GM had a "factory recall" on this problem.