Diagnosis
That's coil whine. It's coming from the internal power circuitry or transformers and is a common, albeit annoying, phenomenon in electronics with high power draw. It's not usually a sign of failure, just a physical vibration in components.
How to Fix It
First, check if the noise changes with network load. Try a simple test: unplug all Ethernet cables and see if the sound persists when it's just idling. Sometimes, ensuring the router is on a stable, flat surface and not crammed in a cabinet can reduce resonant vibration. There's no software setting to fix this.
If That Didn't Work
If the whine is very loud or changes to a crackling sound (which is different), it could point to a faulty power adapter. Try swapping the power adapter with a known-good 12V/3A adapter with the correct barrel jack size to see if the noise goes away.
Heads Up
While coil whine is generally harmless, if the noise started suddenly after a power surge or is accompanied by the router overheating or rebooting, stop using it and contact support, as the internal power supply may be faulty.