if you can film it can be %100 but from the way you describe its dump valve sound so dealer is right its coming from turbo charger.. i dont know the stock turbo pressure on your 550i but since its V8 and upgraded version of that engine with higher boost is being used in M cars its probably lower than 35i's you re driving now.. when the turbo pressure gets higher that sound you re describing increases.. if its something like on the video.. no worries..