Originally Posted by Skorch

I was having pretty much that same exact problem. It sounded like it was coming from behind the seat, around the trunk area or the rear strut bar. It was driving me crazy - I took apart the entire trunk trying to figure out what it was. The weirdest thing was that it happened mostly in the mornings, when it was colder and would be quieter or disappear completely when it was hot. I even took it to the dealership but they blamed it on my tool box/stuff in the trunk and said they did not hear it anymore after taking that stuff out. I took it out, even the spare tire but the noise was still there.

So last night I gave one of my friends a ride in the car and immediately after hearing it he goes - dude, your headliner is buzzing. I did not believe him at first, but after pushing up on the headliner the noise disappeared. I pushed a bit more in several other places and now it's gone completely. I hope your issue is in the headliner as well - good luck!