I was just reading about a certain Grado cartridge. Apparently some phono cartridges aren't happy in the presence of unshielded turntable motors. Many a motor "radiates" an electrical field and the cartridge can pick it up.
I also remember reading a magazine article with Corey Greenberg modding a Rega turntable. He installed some shielding material over the motor, motor compartment, to deal with this sort of problem.
(for Rega 2/3, from an article in Audio 1998 by Cory Greenberg)
The material used comes from Magnetic Shield Corporation and is called Mu-Metal. There is a lab kit that contains several sheets for $80, CO-NETIC AA for low intensity fields and NETIC for high intensity fields.
Shielding the motor with the NETIC didn't work so he tried placing the CO-NETIC sheets on the base under the platter, and this did it.
One can buy directly from the company 630/766-7800; ask for product LK-110.