Hum in an Suminko Project RM-4 Turntable.

I hope somebody has some knowledge to help me out. In my second rig I have an Project RM-4 turntable with a Grado Prestige Gold Cartridge. The turntable sounds very good but it has an small but audible hum. It not caused by a ground. Ive got it grounded six ways to Sunday.

Could it be the Grado Cartridge, I've heard rumors that they are susceptible to motor noise, especially if the motor is mounted within the plinth. Is this true, or just some blarney started by one of Grado's competitors.