The typical sound you're hearing is your car's
starter. The starter is a fundamental part of your vehicle that is responsible for making the engine run when you turn the ignition. When you turn the key, the starter motor engages with the flywheel of your engine, making a grinding or rattling noise. This is the sound of the starter gear interacting with the flywheel's teeth, effectively cranking the engine and getting it started.
Once the engine starts running on its own, the starter motor automatically disengages from the flywheel, and the noise stops. This process is meant to occur quickly, with the starter engaging only for a brief moment to get the engine going. As soon as the engine begins to generate its own power, the starter is no longer needed and retracts, thus eliminating the grinding noise.
