Rear interior squeak?

I'm in the process of trying to track down / fix an interior squeak. I hear the noise every time I hit a pebble in the road or go over a speed bump. I believe I've traced the source back to the interior plastic panel that holds the rear passenger seat belt. I'm not sure if it's rubbing on the rear seat over bumps or if the shape of the panel is just slightly twisting making the sound.

I've tried adjusting the panel and spraying it with some silicone grease that I usually reserve for my door seals. No help. It's driving me mad.

Source of the squeak best I can tell:

 

Are you sure it's not your rear leaf springs? To test, go to each side of the bed, firmly grasp the bed side cap and then push it down a few times until the entire truck is rocking up and down.

 

Hey there. Thanks for the reply. I went out and rocked the truck up and down like you suggested and there's no sound coming from the rear leafs. So I imaging it's got to be plastic rubbing together somewhere.