I needed to start another thread as this issue is more specific. Engine is a 4200 Vortec, I had a few noises before I replaced my rod bearings...some piston slap, as well as worn rod bearings that one seemed to rap slightly after it warmed up. Most of the bearings measured .003" or slightly more and now are right about .0017"..max spec is .0025. That rap has seemed to have been resolved, but I did have a slight knock or hard tap from around the top of Cyl 4 heard through a stethoscope, before the rod bearing change. The light knock went away when I removed the spark from Cyl 4 as heard through the stethoscope when placed on the coil pack bolt. After running the engine for about 2 hours total since the new bearings, the engine is quieter, but I hear a light knock near the valve cover. I took the stethoscope again to #4 coil bolt and there is a somewhat sharp knock and again goes away when the coil is de-energized. What got my attention is that I could easily feel impact of the knock through the tip of the stethoscope. I placed my hand on the coil pack and I can feel whatever is making the knock sound. The other 5 coil packs don't have this sensation, #4 is very pronounced. My question is..could this be carbon build up in the chamber either causing contact with the piston and/or increased compression causing a more violent burn? The fact it goes away when I pull the coil lead makes me think it's a nasty slap and just traveling through the head into the plug well. Was also thinking it could be a wrist pin but why do I feel it in the coil pack? If I place the stethoscope on the block near the top of the cylinder the knock is barely audible, it's significantly more pronounced through the single coil pack bolt. Also very audible when probing anywhere on the coil pack. I have a can of B12, could I soak the piston at TDC and fill the chamber up into the spark plug hole and let it all soak? I tried another water injection but no difference. I have some Amsoil Pi in the tank and after 3/4 tank before the rod bearing change and some idling...when warm it seems ever so slightly quieter but that hard tap at the coil bolt is there through the stethoscope. Anything is possible, all input appreciated.