Just going through the options logically, if it wasn't there before the service it's going to be in an area around the serviced items. Roll up under there and start looking around. If something is loose enough to vibrate against something at higher rpm, it should be easy enough to tell. Start tapping on underbody panels, heat shields, etc. My off-the-cuff guess is a slightly bent heat shield that, when the car is at high rpm, vibrates against something.
Trusting the dealer to fix it is asinine. They did it in the first place. Fixing it would be them admitting to a mistake. Get your hands dirty and figure it out. Honestly, do you want to give the dealer your car with instructions to test drive it hanging out in the top of the tach? "Here, go beat the piss out of my car and see if there's something wrong."
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2011 MB Touring-Sport-6sp-Nav/GTM TT/FI TT TDX/JTran/Kosmic/Eibach/Hotchkis/SPC/CSF/RPS/SoThatsWhereAllMyMoneyWent
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