1987 Ford Bronco

Noises problem
1987 Ford Bronco 6 cyl Two Wheel Drive Automatic 250k miles

when I start my ford bronco II, some times it makes a knocking noise. I let it warm up the noise goes away. After driving for a few miles, 10 or so the knocking comes back and its really loud. The oil level is fine. Today while driving the oil gauge dropped down to the red zone. I checked the oil level, it was fine. Whats the problem.

thanks