Rattling Under Hood

As the engine spins, it turns the main pulley, which is attached to the camshaft. This main pulley drives the serpentine belt. The serpentine belt turns other pulleys, such as your alternator pulley, the power steering pump pulley and others. This provides the power for those systems to operate.

Of course, this does require that your belts are in good condition, and that your pulleys are working normally. Over time and through normal wear and tear, belts will stretch, crack and break. Pulleys will also experience problems (generally due to bearing failure over time).

• Stretched Belt: Belts are made of rubber and while they're designed for high-performance and high-heat situations, they do wear. First, they'll begin to stretch. If you're hearing a squealing sound from under the hood, chances are very good that it's a belt slipping on the pulley.
• Failing Pulley Bearings: When the bearings in a pulley begin to fail, it can produce both rattling and squealing sounds. This can easily be confused for other problems, including a stretched belt, loose exhaust components and more.
• Loose Exhaust: Your exhaust system runs from the exhaust manifold on the engine, underneath the engine, and then under the car to the muffler. If your exhaust is damaged or a support has come loose, it's possible that it could make a rattling sound during operation (particularly noticeable while idling and when accelerating).
• Worn Brake Pads: If you're hearing a squealing sound from the front of the car, there's also the possibility that the brakes are worn out. Disc brake pads are equipped with a "squealer", which is nothing more than a small piece of metal designed to contact the rotor when the pads are close to the end of their life. This creates a warning squeal, telling drivers it's time to replace their pads.