It is just vibration. No worries.
The "singing" or "sawing" sound you sometimes hear from braided fishing line under strain is a result of friction and vibration. Here's a breakdown of how it happens:
Line Movement & Friction: When reeling in a fish or experiencing heavy resistance, the line moves quickly through the rod guides and potentially other obstacles in the water. This creates friction.
Vibration: This friction, especially under tension, can cause the braided line to vibrate.
Braided Line & Sound: Braided lines, being composed of multiple fibers, can be more prone to creating this audible noise compared to smoother lines like monofilament or fluorocarbon, especially with heavier or coarser braids.
Factors Contributing to Sound:
Rod Guides: The material and design of your rod guides can affect the amount of noise generated.
Line Tension: Greater tension on the line, like when fighting a fish, increases the potential for singing.
Line Material: As mentioned, braided line can be more prone to this sound.
Line Diameter: Smaller diameter lines may produce a higher-pitched sound.
Presence of Obstacles: Line rubbing against structures like weeds, rocks, or other objects can also create noise.