Squeak from under rear package trim (aka rear deck), driver's side - 2008 GT sedan

[purefocus]

In hour 2 of a 13 hour drive last weekend a squeak started in the driver's side rear of the passenger compartment in my 2008 GT sedan. Initially I thought it was something to do with my luggage but once the car was unloaded the squeak was still there. It happens over any bump, sounds almost like a squeaky pulley, or alternately a bird chirp or one of those squeeze toys that squeaks. I sat in the back with the seatback down and got someone to drive me around to try and isolate the noise. It is definitely somewhere under the rear package trim (aka rear deck), in the area of where the driver's side rear seatbelt goes up under the trim. But pulling the belt out has no effect on the noise (also tried the belt for the middle seat). The interior is unmodified, no subs or anything.

I found some guides for removing the rear package trim, but before I get into that, has anyone experienced a similar noise and determined the source? Or have an educated guess as to what it would be? It is driving me crazy! And FWIW, it started while driving over an extended section of rough highway.

[Reymando]

You can search through youtube for serpentine sound issues, I've replaced it and the sound is gone. Once you drive it will seem that the sound is coming from underneath the passenger side, but really it's from the serpentine.

Turn the car on, open your hood, spray wd40 on the belt and the sound will dissappear and if not it may be bearings needs replacement (tensioner pulley).

My squeak is gone.

[purefocus]

You can search through youtube for serpentine sound issues, I've replaced it and the sound is gone. Once you drive it will seem that the sound is coming from underneath the passenger side, but really it's from the serpentine.

Turn the car on, open your hood, spray wd40 on the belt and the sound will dissappear and if not it may be bearings needs replacement (tensioner pulley).

My squeak is gone.

Uh, thanks but I was pretty clear in stating the source of the squeak is from up in behind the top of the rear seat. And only when going over a bump. No mistaking it for the serpentine belt under the hood.

[Reymando]

Rear shock mount. Common wear and tear issues, it will squeak from the bumps. I sure someone here will reply to it.

[Thekyleg]

In my experience a bad shock mount is more of a clunk then a squeak. Could be one of a number of bushings in the rear suspension. If you can wait I'll be replacing my entire suspension, including rear shocks and shock mounts. If my squeak goes away, then that might be the first place you want to start

[purefocus]

To reiterate, I have definitely isolated the location of the sound to be under the rear package trim. It's not a suspension noise. My feeling is probably something with the seatbelt mechanism up under the plastic trim, still haven't been able to have a proper look. Thanks anyway.