I think I have the exact same issue...
BUT for me it is caused by the voice guidance being muted. If I am driving and I have a route set in the GPS, at each turn or at any point where the unit is supposed to be telling me to "turn right in blah blah blah..." I just get a low static sound.
I have set the voice guidance to "Mute On" but this does not stop the stupid voice and so i set the volume to 0 and now I get that static.
Hope this helps you diagnose the issue! Now I wish I knew why I can't actually mute the stupid voice!