1991 chevy 2500 engine knock

sound like a rock banging around inside the motor, I don't know what else to do

Oil pressure low?

There's a bunch of possibilities
Loose or cracked flexplate
Defective engine-driven fuel pump (your vehicle probably doesn't have an engine-driven fuel pump)
Defective belt-driven accessory such as an A/C compressor
Bent oil shield or dipstick tube hitting the crankshaft or connecting rods

The most-likely situation is a defective bearing for a connecting rod.