It seems there’s no definitive answer on this one. A couple of observations having had this issue on 2 Boxsters now:
- Sound travels in the cabin so pinpointing the location is difficult. Rollover hoops being in contact with the soft top headliner is what may be causing it.
- Rollover hoops trim is prone to be the source of creaks and rattles which are easily cured by spraying a silicone lubricant between the front and rear pieces (some folk choose to remove the front piece for this). This is what did it for me, I think.
- Failing the above, you could try getting your hand inside the soft top when it’s in half-open position to see what creaks and adding a bit of lubricant to that spot.
- It doesn’t help that this noise seems to come and go with the change of temperature and humidity so not easy to diagnose