Can anyone help? Mechanics are mystified even after road tests and pit inspection. The car runs really sweetly except between 1200-1900 RPM when we get an irritating noise caused by a metallic vibration somewhere in back left side of engine bay. There is nothing loose or missing under the car or in the engine bay that they can see and they have ruled out exhaust shield problems and gear box whine. Car has done 55,000 miles and is garaged and otherwise a joy to drive and own but the noise appears in the most used rev range and is driving us nuts. Any suggestions to look into welcome.