Stihl fs100rx shaft issues

hi l just pick up a brand new stihl fs100rx trimmer and used it for about 10 mins.and owned it for about 2 days And noticed the shaft rattles bad when the trimmer is off, took it to the dealer they told me it's fine then I grabbed another new one off the shelf and it was fine no noise . I asked for a new one they told me they can't exchange it because the serial number is in my name so they will try to fix the one I have
Does any one have any stihl experience or thoughts