I've been getting a buzz in the engine around 2500-2800 RPM, almost like a loose heat shield. The dealer heard it too, they kept the car for weeks looking for the source, eventually a field engineer from Mazda came out with another manual 6 and said both cars sounded the same, and the noise was normal for the manual. I am certain this is not a normal noise, and that the car did not make the sound when it was new. Am I nuts? Does anyone with a stick else notice a pronounced change in the engine sound around 2500 RPM?