My car started making a knocking noise after I gunned it pulling out onto the highway, why?

The engine will shift due to torque when the throttle is released. You may have lost an exhaust hanger and part of the exhaust may be hitting a cross member, frame, or floor boards when the engine shifts.

I believe it's a gas octane problem. Under a load it knocked. Maybe buy gas from a better station or go up to the next pump.