You've definitely got a problem with the starter sprag clutch.
If it chatters the eccentric rollers damage their bearing surface, and get worse. Always.
I had 2 go on my first 916 before I found out the fault was the battery terminal connections causing too bad a connection to reliably drive the starter, which makes it turn slowly and off you go into chatter land.
If its spinning up without engaging it means that the eccentric rollers are getting stuck and not clutching their bearing surface.
Either way means a strip of the left side to remove the alternator and see what condition the starter clutch is in hidden behind the alternator (and if thats on the cards, it means the other nasty, the altenator nut gets checked at the same time anyway
)