My steering wheel has been making a scraping noise for quite a while - to the extent that I don't notice it anymore, but new passengers do. I replaced the plastic bushing with the metal sleeve many years ago (~ 2004 ish).
I always assumed that the clamshell was rubbing the outside ring of my Momo Prototipo adapter. Finally got around to checking it out last weekend and found out otherwise...
The horn contact (Porsche part # 911.613.521.00 - AKA "wiper contact) had a hole rubbed into the brass contact area:
The horn contact was cheap (~$20), so I replaced it.
Additionally, the contact / slip ring / steering wheel cancel ring that mounts onto the steering wheel assembly was nearly worn through at one point:
The OEM part (911.347.099.00) is ~ $90, but it appears that there is an aftermarket part 914.347.099.00 that is the same for ~ $12. I ordered it and will give it a shot next weekend.
One less annoying noise...
Gordo