Have you checked the changeover butterflies in the intake for one with a lost plate or screw?

The best way to diagnose noises is with a mech stethoscope, to help localize the noise. It is very easy to distinguish between a noise in the head and one in the block this way, harder to tell if the noise is main or rod bearings, fairly easy to see it it's a slapping piston from a collapsed skirt. Also easy to see if it's a bad idle bearing.

If it's a main or rod bearing, you should see a marked drip in oil pressure, and with a mech oil gauge with the restrictor removed, you can see a marked flutter in the gauge as the oil hole on the crank is covered more tightly.