Does the buzzing change when the headlights are on? My daughters Impreza had an awful high pitched hum that came thru the xm radion but only when the headlight switch was on. Neither I nor the installer made the connection (sorry, no pun intended) since the headlights are always on. After weeks, I finally noted that the hum went way when the dashboard light control was full on and returned when the dash lights were dimmed. The problem was the antenna crossed near the reostat that ontrolled the dash dimmer and caused an electrical interference thru the XM input to the HU.