Eng Noise, help

I have 440 and I recently noticed an engine noise at idle, 800 RPM. Sounds Like a small hammer is bouncing around inside. Noise disappears as the RPM's are increased
1). The sound has no apparent frequency unlike a rod Knock. At first I thought it was a broken motor mount or the exhaust manifold hitting somewhere.
2). As I increase the RPM the noise disappears
3) My oil pressure drops to 30 psi at 800 RPM and i hear the noise . When I increase the RPM the oil pressure goes to 70 psi. No noise
4) My oil pressure used to never drop below 50psi at idle, 800 rpm, now 30 psi

Should I pull the motor and open it up?
446 CI
mechanical Cam Dur @.050 = 260 and 558 lift
10-1 CR w AL heads