I have noticed recently (since I moved and upgraded speakers) that my SP-15 produces a very quiet, low-frequency hum (much less pronounced than a ground hum). It is almost inaudible when the arm is in its rest, but becomes noticably louder (still less than 1/4 the loudness of a ground hum) when the arm is over the platter (the hum ramps up in loudness quickly over about an inch beyond the edge of the platter). Could this be electromagnetic interference from the motor, or is it something else? I have had to start stacking my turntable on top of my amp (allowing an inch and a half of airspace for heat dissipation - should I do anything else? Add a small fan?) - could this be the problem?
If the noise seems to be coming from the amplifier's proximity to the table (very easy to check), the problem is probably the power transformer in your amplifier. This is not a heat related issue but a radiation issue. The transformer is creating a magnetic field which your tonearm picks up. Try moving the table or the amplifier first before you do anything else.