2000 Dodge Durango Oil Pressure Dropping

4.7 automatic with 90,000 miles.
Oil pressure dash gauge drops to zero after driving a short distance. I installed an external gauge with numbers. At cold start, it reads 75psi. After a short time, it drops to 50psi. When at operating temp, it reads 25psi at an idle. It will stay there as long as you let it just idle. If driven about 3 city blocks, the pressure drops to zero and will stay there until the truck is turned off and allowed to sit for awhile. The engine runs great when the pressure reads zero. No noise, loss of power, overheating, etc. When it is first started, a rattle like noise can be heard. If I reach in and grab the oil filter, I can feel the rattle like it's coming from the filter. I can't dtect it at any of the other components. This stops as the truck warms up. Any help please?