I have a noise in my engine. The sound resembles a wood drumstick lightly tapping the side of the snare. It disappears when I increase revs to 2000 (from idle approx 500). Is it hydraulic tappet? Is it cam chain? Is it piston skirt slap? I've heard a crank-shaft plain bearing go bad but it is not that deep, solid, or hard of a hit. I don't know about con-rod bearing, but I imagine they are similar. There is no loss in power, nor ease of increasing rpm. Any ideas? Oil is Mobil 1 Delvac (Turbo-Diesel-Truck) 5W-40. Used since breakin was completed. It is mostly PAO. Centistokes at 100/40*C are 97.87/14.5.