High performance hydraulic motion control systems typically consist of a two-stage spool valve which controls flow into an actuator. Feedback of load motion allows an electrical signal into the valve to control large amounts of hydraulic power. These servo-systems are well known to exhibit an instability where they “sing” or “chirp” at high frequency. This instability seems to be configuration-dependent. Here, two submodels are proposed that expose a fluid flow induced instability, and a mechanical instability. The result is parameter groupings which expose these instabilities in a fundamental way.