2007 Mercedes Benz SL600 sudden change in engine noise

Engine Mechanical problem
2007 Mercedes Benz SL600 V12 25000 miles

while driving car, engine suddenly started making unusual engine noise. Sounds almost like it is in overdrive. Car idles fast but no pressure on brake pedal. It is a higher pitched hum which follows the acceleration of the engine. Sound like it is working too hard. Dealership says they don't hear anything. Anyone that has ridden or driven the car notices immediately. Do you know what this might be? Possibly a Turbo bearing?