A counterbalance valve, when correctly applied to a hydraulic circuit, modulates the flow of oil when lowering a load, with a crane for example, to prevent an overrunning condition. Under some unique operating conditions, they produce a high frequency, monotone noise that can be characterized as a “squeal.” The objective of the present work is to describe a series of experiments to investigate the flow induced noise in counterbalance valves. The experimental program not only quantified the noise level, frequency, and operating envelope in which “squealing” occurs, but also led to the formation of several hypotheses in an attempt to identify the cause of this noise. These included cavitation, oscillation of valve components, and oscillation of flow within the valve. Analyses of experimental results showed that oscillation of flow is the most likely cause of the “squealing” noise.