1996 GMC Sonoma knocking no oil pressure

Sounds like you have a serious engine problem. Good oil pressure is critical to its survival. Most commonly wear takes place in the engine bearings. That makes it easy for the pressurized oil to run out rather than hold pressure. The resulting low pressure fails to isolate moving parts, which is what the oil is supposed to do, so even more bearing wear takes place. As they wear the very precise clearances between the crankshaft and bearings increases until you can hear the parts banging against each other. The more you run the engine the more wear will take place until something breaks. The constant hammering on the connecting rods can cause one of them to break or the low oil pressure will prevent oil from being sprayed onto the cylinder walls to lubricate the pistons. The pistons are made of aluminum and will scuff and tear apart very quickly. If the engine continues to run it will overheat due to that friction and there will be so many damaged parts that it won't pay to try to rebuild it.

The only thing you can do is to drop the oil pan and all of the crankshaft's main bearing caps to inspect the bearings. They are just curved pieces of flat, soft metal. If you see two different colors, the first layer has worn away and you're down to the second or third layer. Remove those one at a time and be sure to mark each cap so it gets installed the same way. They can't be turned around.

From those bearings oil flows through passages to the connecting rod bearings. Those are more likely to be worn and causing the knocking noise. You will usually be able to move a connecting rod up and down to recreate the knocking sound. A small amount of sideways movement is okay and normal.

If you do find a worn bearing, next look at the "journal" on the crankshaft. Those are machined very precisely and they're highly polished. If any journal is rough or chewed up, give it up. The crankshaft will have to be replaced because that roughness will instantly destroy a new bearing. If you get lucky and there is no journal damage, you can try installing a new set of bearings. You'll want to look for the size on the back of the old bearings and you'll want to use "Plasti-gauge" to measure the clearance so you can buy new bearings that will provide the proper clearance which is critical. Be aware that this would be a last-ditch effort at saving the engine and it will work perhaps 20 percent of the time. The secret is in the condition of those journals.