I dont think it's the exhaust, cause it makes the noise also with the original exhaust. I would say the noise is comming from the right, back corner.
I still need to see if it's not comming from the inside of the tail gate's plastic panel. I have taken every other panel away form the back but I found nothing suspect.
I have already changed the rubber bumpers for the tail gate.
I was also by the local ACL (adac) and they checked it out and said to me that the sway bar's fixation is loosing oil