Mine did the same thing, 8 visits to dealers, finally I had my wife drive and I laid on floor in back. It turned out to be that I needed the seat/rail track replaced. dealer did it, all better. I was/am under warranty so no charge. It would creak whenever i turned corners.
You need tech to sit in back and put his head down by back of seat. he will hear it, and if he lightly touches the rail he will feel it.
RAM 1500 SLT QC 2009 INFERNO RED, ARE Lift Lid ,Spray on Bed Liner, K&N Drop In, Pioneer DEH P8400UB, Alpine Amp, Mopar Kicker sets in all doors and dash, 2 Polk 10" Subs, Polk AMP, SteelCraft Nerf Bars, Mopar Fog Lights, CalTrend Seat Covers, WeatherTech Front Mats, Putco SS Fender Liners, SuperBumper SS Hitch Step/Impact Absorber, Onki Aegis SS Center Guard, PIAA 2100 driving lights