The only thing you can do is lift the bonnet and listen close to the starter when the engine is idling...it will sound pretty loud. I had this happen on an XF Falcon...drove about 500m till car died due to starter finally shorting internally and causing a dead short on the battery which killed both battery and starter motor.
It may just be loose bolts connecting the starter to the engine or the solenoid sticking...no matter what, I'd fix it pronto before it costs big $$$$.
Good Luck.
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