Diagnose a 'Squeaking' noise coming from a wave spring in a dynamic valve application.

Squeaking is a result of stick-slip friction between the spring and the bore/shaft or between the turns of a nested spring. As the spring deflects, the diameter changes ($\Delta D$). If the friction is high, the spring 'jumps' rather than slides smoothly. This vibration frequency falls within the audible range. To troubleshoot, check for: 1) Galling on the OD/ID; 2) Insufficient lubrication; 3) Rough bore finish. Applying a high-viscosity damping grease or using a spring with a 'Gap' instead of an 'Overlap' (to reduce the material that can rub) typically resolves the acoustic emission.