This is one disadvantage of a 'table with an inboard motor. I recall that Grado carts are particularly prone to hum via motor interaction - a problem noted by some users of the Oracle 'tables. If this is the problem (it could also be related to grounding issues as thsalmon notes) then a new cart might help, but a table with an outboard motor will put an end to the issue for good.
Marty
Marty