Here are a few possibilities:
Loose or damaged components: Over time, vibrations and wear can cause various engine components, such as heat shields, brackets, or exhaust system parts, to become loose or damaged. These loose components can create rattling noises. If the sound suddenly went away, it's possible that one of these components finally came completely loose or fell off.
Worn-out or damaged bearings: Bearings are used in various parts of the engine, such as the alternator, water pump, or pulleys. If a bearing becomes worn or damaged, it can produce a rattling sound. If the bearing completely failed, the noise could have stopped as the faulty part ceased to rotate.
Engine belt issues: The drive belt or serpentine belt in the engine can become worn, cracked, or misaligned. A damaged belt may produce rattling noises as it slips or makes contact with other engine components. If the belt completely broke or came off, the sound might have disappeared.
Internal engine problems: In some cases, a rattling noise could indicate internal engine issues. This could include problems with the pistons, valves, or connecting rods. If the noise suddenly stopped, it could mean that the problematic component completely failed or ceased to function.
To determine the exact cause of the rattling sound and what broke, it's best to consult with a qualified mechanic. They will be able to perform a thorough inspection of your car's engine, diagnose the issue accurately, and recommend the necessary repairs.