I have heard the same but no personal experience. I have also heard that by grounding the motor directly (ground wire from motor to groung) will take care of issue.
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It is true. I have a Grado and use it in an Oracle TT/MMT arm combo - the motor is shielded, no hum! Same cartridge used in a Mission TT, same arm, with unshielded motor - when the cartridge is on the outside edge of an LP hum is minimal but as it moves toward the center where the motor is the hum increases and becomes fairly noticible.
It is possible to jerry rig shielding of the TT's motor (which I've never done) but it is just easier to get a different cartridge which is shielded.
This seems to be common with the pro-ject turntables especially if your cartridge has good low end responce. The problem seems to be in the motor suspension. On my RM 5 I discovered this by placing my finger under the plinth below the motor and just slightly raising the motor so as to eliminate contact of the motor with the plinth. The motor suspension as designed, consists of 2 Sorbatane pads and a rubber band, if you will, that is screwed to the plinth. I believe the problem is mainly with the band making contact with the plinth. Vinylengine also has a thead regarding this issue you may wish to check it out.