Thanks for the suggestions so far.

No I didn't change the oil pump when the motor was rebuilt (because was still showing good pressure), sounds like a good starting point

I should have said that on the highway (3000rpm) pressure is 50-60 psi and once I drop speed (and revs to 2000rpm) presure drops to about 40psi but the noise goes within 20 seconds. And I get no noise unless I'm hitting highway speed. Around town and 80 zones are fine, engine nice and quiet.