I've heard this 3-4 times in my career with different brands of machines. The last an Engel 1,000t Duo that did it with EVERY mold it ran regardless of resin, gating, valve gates, and water temperature. Another was a Toschiba 500t hydraulic clamp. It never seemed to make a difference and I know the Engel is still running. Not sure what to tell you or how to stop it.
Rick.