Question, why would you redline your car for ten seconds? Are you sure its engine speed related. First you need to rule out the possibility of accessory damage. Remember, some of your accessory pullies are turning at 3 or 4 times faster than the crank. 21,000 RPM for at least ten seconds is not exactly easy on pully bearings! Take your belt off,start the car, and if the noise is there, you have just ruled out accessory drive damage. You need to get access to an engine ear or some kind of stethoscope, this should be a snap to diagnose. You said you have taken it to some shops? And they could not give you a diagnosis? I'm not buying it.