A buddy of mine tried replacing his pickups in an guitar he had laying around. It's (2) humbuckers, (2) tones, (push/pull on all 4), (1) 3 way switch. Attached is a pretty close wiring diagram other than the 3 way switch. As far as I can tell with my continuity tests, it's wired up correctly, and the guitar operates normally, other than the hum/buzzing once one of the volume pots are turned down. The pots all the way up, the guitar is fine and plays fine.

Any clues is appreciated.

 

The only logical explanation for what you describe is that the amplifier input impedance is reducing as the volume is increased – this should not be the case with a well designed amp.

With the volume pot set low, the input impedance will be that of the pot, but once the volume is increased the input is loaded with the amplifier input stage in parallel with the pot.

If the above is the reason for the hum, then loading the guitar output with a suitable resistor should eliminate the hum. I suggest you try with a 47KΩ resistor as a start – for neatness the resistor could be mounted within the guitar.

 

Double-check your grounding arrangements. If the strings are properly grounded via the bridge, putting your hand on the strings shouldn't reduce the hum.

 

It's possible that the ground and inners have been swapped by mistake.