Jolida JD-9A Phono Stage Hum Problem

I have tried all the settings on the rear and both outputs, put cheater plugs on all the gear in all possible combinations, tried different interconnects. This a 2 channel stereo, no video connected. There is a low level hum (sounds like a 50 or 60 cycle hum) that none of the above has any effect, however if I grab, with my hand, the case where the transformer is located (left middle), the hum is reduced by 90%, when I let go, the hum returns. Short of holding the preamp when listening, does anyone have any ideas?