Steering System Noise Definitions: Whistle
WARNING: This page is about a different car, the 2007 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
Whistle is similar to hiss but is louder and of a higher frequency. It is also more of a pure tone noise than hiss. Whistle is air-borne and is generated by a high flow rate of hydraulic fluid through a small restriction.