1997 Ford Mustang lifters tapping real loud

I have a 6 cyl. mustang and the night before it was just fine and when I started it in the morning the top end of the motor seems like it is not getting oil, the lifters are real loud and the top engine is real loud, I took the oil pressure sending unit off and pressure checked the lower part of the motor and it seems to be fine, What would cause me to get no oil to the top of the motor but yet the bottom of the motor is just fine????