An air bypass valve is a valve in the cold-air induction system that is designed to prevent the system from taking in water. If a vehicle becomes submerged or goes through a large rain puddle, a faulty valve can let get into the engine and prevent the cylinders from fully compressing. If the cylinders cannot compress, they must shut down to prevent severe engine damage.
Screeching in Engine
If the air bypass valve is starting to fail and moisture gets into the engine cylinders, the cylinders can't compress, and this can result in the warping of several of the engine components. The cylinders and other pieces are no longer able to work in conjunction in the way they were designed to, and they will start to collide with each other as the pieces start to warp. The result of the hot metal parts scraping against each other is a screeching sound coming from the engine block that indicates severe engine damage.
If the air bypass valve fails at low speed, the engine can just die outright without any lasting damage. The individual components of the engine would need to be dried before the engine can function normally again. It is more clearly the air bypass valve if this happens right after driving through a puddle or if the engine is inadvertently submerged.
Too Fast an Idle Speed
A faulty valve doesn't only allow water into the engine but air as well. The resulting vacuum leak allows excess air into the closed-pressure system of the engine and affects the fuel-to-air ratio that the engine needs to maintain combustion in the cylinder. The added air results in a higher idle, and the engine will not revert back to a low idle. This can result in engine damage over time, and the engine will need to be looked at by a mechanic.