So...I had some free time this afternoon and dove a little deeper into the white clamps that hold the seat bottom to the seat frame as depicted in Clarks images a few posts ago. I was wrenching around on the bottom of the seat to try and make it replicate the noise/thunk and I was able to get pretty close to the actual experience.
Low and behold, if I kept one hand on that white clamp at the front of the seat just on the underside I felt it move/vibrate in unison with the noise. I am still a little weary because for me it always sounded like the noise/movement was coming from the back of the bottom seat cushion. However, I have a temporary fix to try and isolate this movement to see if that stands the test of time.
I put hose clamps with rubber on the inside diameter of them on either side of the white seat clamps on the same tube the clamps wrap around (a total of 4 clamps with one on either side of each white seat mount). My purpose is to hold the white camp stationary and it seems to be working well so far. I took it for a good run around some of the roads that always cause the seat to move and nothing happened! However...time will tell as I have thought I nixed this one before..
It is kind of weird because it looks as if there is grease between the white seat mounts and the bar they are mounting/clamping to. Why would you want to grease something you are trying to keep stationary? I had my seat frame replaced once and it worked for a week or two...but came back so it seems these camps wear out after a little use and getting in and out of the truck. I have to say that replacing these white clamps that mount the seat bottom to the frame would be pretty easy for GM to do and cheap to assuming they can find something with a little more clamping force to stop the seat from moving.
I will report back if it comes back any time soon