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?
If you are in Britain, it is probably 50 cycle hum, if in the US, most likely 60HZ. Is your hum ambient, through the room, or is it amplified through the speakers? You may just try tightening the mounting bolt that secures the transformer, or you loosening the bolt, slipping a bit of damping material under the trannie and tightening it back up. Make sure that the material is not thermally reactive.
Try some weight on top of the case in combination with some type of rubber material above where the transformer is located. See if that improves things. As an example, I use 40 durometer rubber squares on top of the transformer cover of my amps, then place VPI bricks on top of that.
If you can touch it and the hum more or less goes away, it's probably a grounding issue near the transformer. It might be a loose ground wire or lug. As mentioned above, try locating the transformer inside the unit and tightening everything around it.
Thanks for your responses. I am in the USA and it is not ambient, amplified thru the speakers. I have tried weighting the top of the unit and surrounding the case with metal, nothing stops the hum but my bare hand grasp. As I recall, the (4) transformer bolts, attaching the it to the case have rubber washers to isolate it from the case. I'll pop the top and look for loose connections ....
Well, all the ground wires end at a bolt in the middle of the bottom of the chassis. I took the nut off and removed the "O" lugs and found that there was only a small amount of powder coating removed and the bare metal had some surface rust. I removed the rust and created a larger, powder coat free diameter for an improved connection. This did reduce the hum by about half, but it is still there. Funny thing is, grabbing the chassis near the transformer no longer has any effect at all, no reduction of hum. Does anyone else have this problem?