Hum In Turntable

I am selling my Rega RP-3 that I bought back in May ( good table) but it just doesn't fit in my system. I just got a one owner (my 3rd Linn LP12) with the Balck Ittock LVII tonearm. There seems to be a hum that increases as the arm moves inward toward the spindle. It will increase with volume but can't be heard with music playing. Any ideas? I've never had this problem with a Linn. This one does have both grounds connected to the sub chassis and not to the arm board screw, and I'm going to try that tomorrow. I'm open to any trouble shooting ideas. Thanks
what cartridge? Some cartridges (particularly Grado) pick up 60hz from AC motors - behave exactly as you describe.
Bdgregory, thank you for your reply. There were 2 ground wires going to the subchassis so I hooked one up to one of the armboard screws. No more hum. Thanks for the info about certain carts.