My name isXXXXX am not a Porche Specialist but I hate to see you sell a perfectly good car because of a lousy rattle. Over the years I have chased down many rattles and usually it ends up being something fairly conspicuous. And I say to myself " Gee Why did I not check that earlier" But automotive diagnostics being what it is makes things like this extremely difficult to isolate. You have what we call a short to long term intermittent noise. That makes it even harder to find. For the record I have had vehicles that have had a shock absorber that would rattle inside but you could not find anything wrong with it. The way I isolated it was to switch shock from right to left and see if it moved. Sure enough it did. A shock absorber can make a noise that will make you think the wheel is going to fall off because it transmits the internal problem into the frame or sub-frame. If I had a bit more information to go on I might be able to help you but this is one of those problems that takes a lot of patience and time to isolate and usually most technicians have neither.
Try to think about when and how the noise develops. If it is the same all the time when it does it. If it changes depending on whether the vehicle is having G force applied in a curve for example. Anything that can give me a clue as to where to look. Check the shock towers to make very sure that the 3 nuts that hold the rubber bumper on the top are secure and tight. Look for sheet metal that may be vibrating against something like the exhaust. I have even run across Catalytic converters that rattle pretty bad when they get hot because a weld has come apart inside the converter.
These are some of the possibilities that it could end up being. It really comes down to whether you have the time and patience to deal with running this problem down or whether you feel you would be better off to swap it out with the dealer for one that does not have the problem. Only you can decide the answer to that question. I will try to help you as much as I can from this standpoint but it would be so much easier if I had the vehicle where I could play with it and get it to produce the noise while I was either driving it or by simulating the vibration one way or another.
I hope I have helped you a little with the issue.
PS-I am disabled and I subsidize my income by the earnings I get from my customers. I am available about 50% of the time and I must rest when I am not working for you.
Thank You for accepting my answer but I would like to try to help you resolve the problem if possible.