I have the exact same problem, 2015 Macan S, 52k miles. Twitter and chirping at 1200 rpm. Regardless of gear.
Also, took the Macan S to local Porsche dealership. They investigated and found one of the flat welded ties on the mid-pipe to muffler assembly to be cracked. The tie is just aft of the connection clamps to the down pipe, and ahead of the small cross/h-pipe. There is a duplicate welded on top of the pipes that is not broken. Porsche said if the car was still under factory warranty, they would replace the whole mid-pipe to muffler assembly. Cost would be $3500. Service advisor knew I did not want to spend that, and said I could just have the tie welded.
Took it home, and saw the tie was redundant and not really necessary, so I cut it off. Noise still occurs, so it was not the tie.
Took to pals house and put on lift. Put in gear and ran at 1200 rpm, in gears 1-6. Heard some of the noise, but could not get same intensity of chirp/twitter. Noises were coming from various places on exhaust pipe, but mostly from down pipe flex joints. At 1200 rpm the whole exhaust shakes, much more than at any other engine speed. Definitely related to harmonic vibrations at that rpm.
So, does anyone know of a TSB for the downpipes?