What you are describing sounds like a ground fault in either the tonearm or the table. Start the process of elimination. Disconnect the cartridge and see if the hum is still there. If so, touch the table spindle and then the tonearm to see if things change.
Phono Hum w/ Pro-Ject Debut III
I am experiencing phono hum with my Debut III. Phono leads are going into my phono inputs in my Sunfire Theater Grand II. I think I am getting AC feedback from cable routing in the back of the system. The ground wire is connected from the TT to the Sunfire. I started moving the phono leads away from the AC lines and the hum has lessened but it is still there. Is there a good way (material) to shield the phono leads without changing out the leads themselves?