HELP! Squeak / creak from rear of 03 Cobra

[mikepol2]

OK this is driving me nuts...I have an 03 Cobra and when the body flexes, there is a loud creak noise that seems to come from behind the passenger side rear seat. I took all the trim out of the trunk and finally found that putting pressure on one of the sheet metal supports behind the seat made the noise, so I stuffed a rag between the sheet metal pieces and the noise went away. You couldn't hear it outside the car, only inside. But now it's back! I re-did the fix but this time it didn't work. Before it came back, I replaced the spring isolators, LCA bushings, subframe bushings (what a PIA but best results of any other mod to the car), and sway bar bushings with urethane, and greased the crap out of them with the Prothane white sticky grease. It was fine for months and I don't think it's suspension components. Only seems to happen at low speed. There is also a single squeak that happens when you go up or down speed bumps, and hit certain bumps in the road. That happened since before the suspension mods too and never went away, and I'm not sure but also think you can only hear from inside the car. Any ideas would be appreciated...