Andy amg

Hi guys i'm new to the forum. I have had my GTS for 5 years now it has been remapped and has a Renntech exhuast which was installed over 2 years ago, but recently it has developed a low RPM judder which you can feel through the seat when stationary. The problem is covered up if i drive it in manual with the paddles and keep the reves higher (above 2.5k rpm.) It went to my Merc main dealer for a service and it started not long after that. The dealer sent 2 tech guys out in it with it plugged in and found nothing but they did admit they could feel the shaking. The car has no codes recorded and isn't showing an engine light. Mercedes has said they would need to take the car away for an investigation and haven't given me any idea of the cost so i thought i would give this forum a try and see if anyone has had this and could maybe shed some light on the issue. Any help would be appreciated and Thanks in Advance