sounds like something is binding in the linkage.

take the servo arm off and move the steering manually... if you feel resistance near the center comming from the left then you need to eliminate it.
do this and it should be fine.

also a tiny bit of servo hum isnt all bad.. it just means that the servo is working on centering and its almost got it but not quite. (usually due to stiff linkages)

if left for extended periods of time in that position though it will drain you RX pack.

it shouldnt matter much on a MT though.