The K8400 housing acts as a good resonator, so you need to minimize the vibrations. The Z-Axis is a major noise source, therefore I printed a z-axis holder to minimize the z-axis vibrationshttp://www.thingiverse.com/thing:635751
Another noise source is the fan on the bottom of the printer. You may try to get a part which has a little bit better bearings, but for this part I have no real solution yet. Maybe it will improve using an air flow guide, but I didn’t try it yet.