SLK350 2005 R171 Single Knock on startup
 

Hi All
My slk350 88000 miles with full dealer & specialist service history. If I park the car on an upward incline nose first and start the car, I get a very noticeable single knock coming from the front. It feels like a single blow from a hammer hitting the car inside the engine bay, you feel it through the brake pedal. Only does it once, the car does not burn oil, oil level correct, engine runs fine. If I turn the car around and park nose down no knock at all. Merc specialist has checked engine mounts, suspension bushes and exhaust mounts with no issues. Has anyone experience this before or can help identify the cause. The engine is showing no fault codes.
Thanks