Eng Noise, help

EXACT SYMPTOMS

446 CI
Mechanical cam @.050 256 duration 558 lift

My oil pressure is always around 70 psi from idle to 5500RPM, fluctuates 5 PSI the whole range-NO NOISE

Sometimes when I idle my pressure drops to 30 psi and I hear the erratic hammer noise coming from the center of the engine.
Right now the car is idling at 800 RPM 70 psi no noise
WHY it suddenly dropped to 30 psi at idle then returned to 70 psi at idle I do not know?????

I have header . Not a heat riser rattle . the noise is a heavier hammering. Just like a heavier hammer head was bouncing around but only at 30 psi