BMW X5 engine makes abnormal noise when working normally

Q: 2010 BMW X5, when the engine is working normally, there is an abnormal noise of “da da da”, and the abnormal noise is more obvious when releasing the accelerator pedal. The abnormal noise has not changed in the cylinder-by-cylinder fire test. How to diagnose next?
A: The car has a fault phenomenon of “da da da” abnormal noise when releasing the accelerator pedal. If there is no noise when pressing the accelerator pedal or holding the accelerator pedal steady, it may be a malicious noise, and the cylinder is often strained. If the sound of the fire test does not change, the possibility of cylinder pulling cannot be completely ruled out, because there may be multiple cylinders pulled. During auscultation, if there is an abnormal noise when pressing the accelerator pedal, and the abnormal noise is more obvious when releasing the accelerator pedal, it may be that the transmission belt has cracks, and abnormal noise is generated when it is running to contact with the tensioner. The transmission belt should be checked in particular.