It is most definetly a grounding (earthing) problem

Perhaps the strings are not earthed ( connection on the bridge )

OR it could be any other metal component

a good trick is to open up the electrics and plug the guitar in- so it's humming

Then with a length of insulated wire, touch one end of the wire to the jack plug body and touch the other end to every metal component

When you find the right component the humming will stop

If you've reversed the polarity on the jack socket - this will also cause a hum