weird buzzing/ humming noise

[sassanbehnam]

Hey guys,

I'm new to the forum, but not new to MBs. I picked up a 2004 G500 with 106k miles yesterday. when I started driving it a bit more,
I hear a weird buzzing/ humming noise coming from behind the rear-view mirror.

here are some characteristics.

-Sound starts at 45MPH, speeds any lower than 45mph and you won't hear it.
-It is not affected by engine speed, changing gears or shifting into neutral does not change the sound/volume/tone.
-Once you hit 45mph it increases in volume and remains constant.
-If I rev the motor in park, there is no noise.

Here is a short video clip I shot.

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[optimusprime]

Take a look at the base of the mirror, do you see aet of slots ,,,,on some there is a sensor in there for the aircon .I use a vacuum sweeper to suck out any fluff .