I had a rant on another thread about what this _might_ be ... which I have suffered from twice now, on my old 916 and my recent 748.
Before I launch again ...
DONT DONT DONT keep trying to start it if it chatters on the left side - buzzing on the right means the relay isnt getting enough current to engage fully -
chattering on the left means the relay is engaging, but the starter is only getting enough current to take up its slack, and that will hammer the sprag clutch and kernacker it pretty quickly, requiring left side surgery !
Anyway - the poxy culprits of lots of 'is this battery duff ?' questions are the little right-angle brackets on the battery terminal - they have a threaded bolty-bit (technical term) splined into the sheet, which can slip if you tighten it - and then you've got a not-quite connected terminal, which drops enough volts to stop the starter
and often confuse a 'smart' charger.
It goes without saying that you've checked the bolts on the top of the battery for tightnes of course
