Cracking (probably) from the interior

Moin,

I recently added a beautiful 3.7L tiptronic quattro with 164,000 km on the pointer. I didn't notice anything at the test drive. But already on the transfer journey it started with an occasional snare/cracking, which I locate in the interior in the area driver's back height lumbar spine or behind it. But it can't be exactly located. I've tried everything possible to chase the (partially electric) seat through all positions, several times. Shaken at the seat, etc. In addition, I have to note that the noise does not occur in the stand, so I can jerk back and forth in the seat and drop myself on the seat - nothing. It needs to be driven, in case of uneven road conditions, when torsion into the ground. comes, the sound is heard. But it's not clean to reproduce. Will the vehicle now enter the workshop, because anyway the light range regulation has failed again despite repair before purchase and hope that the workshop finds something. Because otherwise I'm selling the car again, I'm afraid it's such an infinite story that can't be determined cleanly. And an A8 that cracks or crunches, is going out for me. Unfortunately, the car is otherwise phenomenal. But maybe there's someone here who can help with a tip. I'd really like to keep it.

Greetings.

Andreas