Car hesitates and then makes grinding noise when starting

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My Z4 has been hesitating upon start and then makes a grinding noise as it starts. This lasts just a few seconds, no more than 10. It doesn't do it every time, seems to happen only in he mornings but later in the day when I start it again it seems fine. Had the battery checked and the alternator both checked out. Took it to a shop but since they can't hear the problem as I already started it they don't know and say it's starts fine, which it does sometimes.