Old story. We had some hydraulic valves in a control system that would 'scream' under certain input conditions. The techs had changed them out, and the noise

Old story. We had some hydraulic valves in a control system that would 'scream' under certain input conditions. The techs had changed them out, and the noise went with the valves. Vendor was positive that it could not be the valves having a resonance in that range. Borrowed an HP audio spectrum analyzer, and played back a cassette tape of the noise into it. Sent the results to the vendor, and they were able to duplicate. HP Test Equipment FTW.