What would make the rear hum in a 2007 Pontiac Solstice

What is your question?

I have a 2007 Solstice, with less than 21,000 miles that the rear is humming. I have not driven the car in almost 3 years because of this! When I had it to the dealer in 2016, I was told by the mechanic that this is normal. To me, if it was normal, I should have heard the humming from day one. I was also told, the gears cannot be replaced but that the whole rear would need to be replaced...is this true? There is no leaks.

What makes this problem better or worse?

Is more noticeable when the top is up

How long has your 2007 Pontiac Solstice had this problem?

Noticed in 2016, haven't driven the car since because of being afraid to cause more problems