What does a squealing noise that increases with engine speed usually indicate?

A squealing noise that increases with engine speed typically indicates a worn or loose fan belt. As the engine RPMs rise, the belt spins faster, and any wear or looseness will cause the belt to slip on the pulleys, producing that distinctive squealing sound. This sound is often more noticeable under acceleration or when the engine is under load, as the increased speed amplifies the effects of any slippage.

The other options point to different mechanical issues that do not generally result in a squealing noise linked directly to engine speed. For instance, faulty spark plugs would lead to misfiring or rough engine performance but not a squeal. Low oil pressure primarily would affect engine lubrication and may lead to knocking or tapping noises rather than a squeal. Overheating engine problems usually manifest as steam, hissing, or other sounds related to coolant issues, rather than a consistent squeal that changes with engine speed. Therefore, the presence of a squealing noise that correlates with increased RPMs is a clear indicator of a fan belt issue.