WHISTLING on the engine!

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hi guys! my '07 e70 3.0 diesel started whistling a week ago. the higher the rev, the louder the whistling. it was like an ambulance. initial diagnosis of the dealership was the turbo but werent really sure. a week has passed and still NO DEFINITE DIAGNOSIS. they're now inspecting the whole exhaust system to rule out any other problems. they said if it's the turbo, i'd have to replace the whole assembly which cost like $3500. any suggestions as to what the problem is? or if need be, should i replace the whole thing or just a part of it? thanks guys.