By touching the arm you are causing the so called ''ground loop''.
There must be something wrong with either your phono-cable
or the tonearm ground. I own + 40 carts with no hum problem
at all except by my most expensive cart; the Allaerts MC 2 gold.
The problem must be in the cart but by Expert Stylus in UK they
were not able to discovere anything wrong with the cart. No
wonder nobody knows how and why hum problem arise and
conseqently how to get rid of it. ''Trying'' is the only advise one
can get.