grado hum

While checking for ground loops or other grounding problems is worthwhile, Grados often hum if operated within the range of the motor field.

Changing to a cartridge other than a Grado is one option but you can also look for shielding material to apply to the bottom or your platter or around your motor. Someplace there is a magazine article on modifying a Thorens or AR table to allow use of a Grado but I don't recall where.

Roggae, for information on shielding your motor, Google "mu metal" for information and suppliers. Here is one example -

http://www.mumetal.com/

Modifying your table should be much less expensive than buying a different cartridge, AND you will preserve the characteristics of the Grado you enjoy.