Chuck,

Could there be some leakage from the phono motor coil to the chassis?

This type of problem can be maddening, and is usually of simple cause once discovered. I had a similar problem with a modern stereo magnetic cartridge. Turned out I had accidentally reversed the ground connections between the left and right channels at the cartridge terminals. Another time I had hum being magnetically injected into the tube of a preamp because it had been mounted too close to the motor!