What's causing it is that the pickup cover is acting like an antenna for noise (mostly from your body) unless it is grounded. Some pickup manufacturers automatically hard wire an internal lead from the pickup casing to the ground lead from the pickup so that this isn't an issue (Darkstar pickups are wired liked this, for example), but for some reason Hanson failed to do that with the Chi-Sonics. Some pickups will buzz in a similar fashion if you touch the polepieces- this can also be solved by adding a strip of copper tape across the bottom of the polepieces and grounding the strip. Again- some manufacturers incorporate this into their design and some don't!
