I was just thinking about the noise. These guys are most likely on the correct path with the piston clearance. I just wanted to add something different. The oil restrictors would be in the back of the block and screw in beside the cam plug. We use them all of the time to keep the majority of the oil in the bottom end where we need it the most. If they were used on a hydraulic lifter they might starve the lifter to the point that they dont pump up and make noise. This might also explain the trickle of oil at the Rockers. I am curious to hear what kind of oil pressure you have. Keep in mind if the restrictors were used that it would not show up on an oil pressure check. I have some ideas if your oil pressure is low. but i will wait to spout off until you know what it is.