Generically, sounds like a ground problem, but that is consistent with the more detailed answers above. Two data points, may or may not help, but more info:
1) Added aux-in on my Harley, got that buzzing, and ground loop isolator was quick/easy fix
2) I have an external Belkin bluetooth adapter on my X30 that plugs into the Alpine's aux-in, and sometimes that mini-jack gets jarred around in transport or on the water creating high-pitch whine. Unplug, plug back in, all is well.