The main noise you hear under the hood is that of the engine’s pistons going up and down inside the engine’s cylinders. Powered by the mixture of fuel and oxygen, combustion occurs when the spark from your spark plugs act as the trigger. As you accelerate the pistons move faster and the noise changes appropriately. When the timing is off or when engine calibration is an issue or worse when there is insufficient oil you will hear changes in what would be your regular benchmark engine sound. You may hear tapping, irregular piston firing, or a metal to metal slap. At that point stop operating the car and get it to the mechanic ASAP.