92 van motor knocking

Hi, i've a 92 chevy van 305 V8 motor. I recently have rebuilt the head and 2 pistons, about 2 month ago. for about one month, the motor start to knocking very hard to noice, last day i put in the oil 1 liters of a product who is suppose to reduce the gasket and cylinder noise, but about 10 km after i put it the noice came more louder. I guess that the product that i put in the oil have fuc**ed up something, when I'm at a light the motor sound like news, no noise, and when i drive very slow, 5 mph of less, same thing, it's when i accelerate that the noice came out, i've thinked my head gasket have trouble or the piston or mayby the manifold are cracked ( but it sound more like the motor knocking) thanks you for your precious time...

Sounds like the bottom end is cashed out, only a visual inspection will tell.