The rooftop is not closed properly or detected malfunction during closure cycle. It has to go through full operation, including closing the windows fully. The expensive solution and ultimately, the better one is to find a facility that can calibrate the roof. If you do not, the system can become completely misaligned and the bars can brake, needing a welding repair, etc. A temporary solution is to have someone help you pressing the button and go through full open and close cycles, while you inspect the roof. You can try gently pushing at either sides of the back panel assisting the closure, before the back window drops down. If the roof is aligned and fully closed, the pinging will stop, otherwise it will go on as long as you are driving and you will have to invest in a good sound system to block it out

. It is still the best to calibrate the roof as soon as you can. I hope it helps.