Driver's Seat "thumps" under load
 

I know this sounds strange but when I am driving and either come to an abrupt stop or accellerate quickly the back of my seat seems to gently "thump" me in the back. It's almost as though a breifcase were in the footrest of the rear passenger compartment and hits the back of my seat. I have gone over it head to toe but have found nothing. Does anyone know what the internal components of the backrest look like? Could something be loose within the seat itself? Is there any way to "tighten" the backrest? I have power drivers seat 98 C230.

My problem is not so much of a "click" but more of a subtle "thump" in the middle back only with abrupt stopping or starting. If any of you have paperwork addressing this from a stealership describing it as a "defect" (along with labor involved/expense) I would love that ammunition as my car is no longer under warranty. I have checked alldata.com but to no great avail other than attributing it to "Various noise sources at contact points and surfaces on moving components in the seat adjustment mechanism cause noises under load and must be identified separately for each complaint." :confused:

If it is not the entire seatback thumping, but mostly the center, it could possibly be the stem/rail mechanism that supports the headrest (just a guess here)
It functions like a ladder of sorts - perhaps it's loose on the bottom - so combined with the sudden pressure of the drivers head against the headrest it pushes it... etc etc.

good luck