This sound could be thrust bearing wear. What happens is the crank walks back and the counter weight hits the rear of the block at the pan rail.
I have seen this on a couple motors especially when heavy metal slugs are welded in for balancing. The welds are never cleaned off properly and tend to hit with just a small amount of play. This noise will come and go depending if you hit the brakes hard or accelerate hard.
Easy check is move the crank front and back and measure the distance it moves. I'm guessing you will find to much play. Replacing the thrust bearing is easy. I know guys that just put the cap side only in. But both can be done in the car with just lowering the rear of the crank.
Pan can be dropped in car with 1 and 2 counter weights up. Otherwise the pan will hit coming out in the front.
