My turbo has started making a weird noise, kind of a growl with a pop, sounds like prop cavitation. It only occurs when the turbo gauge registers in the neutral zone at the center of the gauge. When I floor it, it responds fine with no noise (other than that highly satisfying whoosh), most of the time--although occasionally it will not move off of the neutral point. At first I thought it was a loose heat shield, but shaking things under the car does not reproduce it. If it made the noise all the time I would suspect bearings. Has anyone else experienced this? I have only had the car for 15K miles, so I do not know its history.