What is a likely cause of a vehicle having a squealing noise that changes with engine speed?

The presence of a squealing noise that changes with engine speed is most commonly associated with the condition of the accessory drive belt. A loose accessory drive belt can slip on the pulleys as the engine speed increases, which results in a noticeable squeal. This noise fluctuates with engine speed because the rate at which the belt slips—or skids—on the pulley directly corresponds with how fast the engine is running.

The implication of a loose accessory drive belt is that it isn't maintaining adequate tension when subjected to varying loads, causing it to make noise and potentially lead to further issues, such as a loss of power to the accessories driven by that belt.

While a worn-out fan belt might also produce a similar noise, the primary issue with a worn belt typically would be that it can become frayed or damaged over time. A faulty alternator can lead to unusual sounds but is less likely to create a noise that consistently changes with engine speed. An imbalance in the tires would cause noises that are related to wheel speed rather than engine speed, making it irrelevant to the squealing sound described in the scenario. Therefore, a loose accessory drive belt stands out as the most likely cause of that specific noise behavior.