4.3L noise

OK i'm at my wits end on this one.
I have a 1995 chevrolet G20 van. It's equiped with a 4.3L TBI V-6. I have a knock in the engine that shows up after its warmed up. If i were
to describe it, it sounds just like a lifter. I have replaced the lifters ( very $pricey$ i might add, its a roller lifter engine ) with no luck. This engine is equiped with the counter balancer by the way. After doing that, i removed the inspection plate from the transmission, unbolted the torque converter and slid it back out of the way. I fired the engine up and the noise was still there. Next i removed the serpentine belt and fired the engine up and the noise remanined. Fustrated i searched the internet and i came across a GM Technical Service Bulletin that said the exaust valve seals might be starving the guides for oil causing the valves to bind. I went and purchased the updated seals from GM, installed them, and noted no change.
The source of the knock seems like it is coming from the center of the engine. I placed my stethoscope on the timing chain cover and i can hear it. I have heard tales that the noise may be caused by slack between the counter balancer and camshaft gears and supposedly GM knows about this.
Its not a deep knock like a rod or a main and like i said earlier it shows up after the engine is warmed up. The engine runs great, very smooth, has good oil pressure, produces good power throughout the RPM range and it only has 60,000 miles on it. The engine has been treated to regular oil changes.
Thanks in advance!
PMB
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Could be piston slap or even a noisy injector.
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My 2003 has 130,000 km and I have noticed a similar noise. Actually I know the difference between a lifter noise and a piston knock. It seems more like a slap or lifter. What should I do? Add a special oil ?

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