Mazda Repair: 05 Mazda 3 slight rattle, sympathetic vibration, mazda 3

Question
Before the car automatically shifts into higher gear there is a rattle noise from under the hood. Sounds like a small rock rolling around, but when it goes into higher gear it clears up. Car has 173k and runs great otherwise.  

Answer
What you are describing is called a sympathetic vibration. It is probably just a heat shield or similar item that rattles when the vibrations from the engine reach a specific frequency. If you need to have the problem fixed, a technician can put the car in the air or on jacks and hunt for the origin of the sound while another person changes the rpm of the motor.