Diagnosis
That's coil whine from the router's internal power components, specifically inductors or transformers on the main board. It's a physical vibration caused by electrical current, and while generally not a sign of failure, it can be very irritating.
How to Fix It
Unfortunately, there's no software setting to eliminate a physical noise like this. First, check if it's coming from the router's own power adapter brick as well. If the noise is only from the router itself, try relocating it. Sometimes placing it on a different surface or ensuring its vents aren't touching anything can dampen the resonance. The noise often changes with network load, so you might notice it more during heavy downloads or data transfers.
If That Didn't Work
If the whine is severe and constant, and the router is still under warranty, contact Asus support. They may determine it's excessive and offer a replacement. Some units are simply louder than others due to component tolerances.
Heads Up
Don't try to open the router and fix it yourself—you'll void the warranty and risk damaging sensitive electronics. The noise is a nuisance, but it doesn't typically affect performance or longevity.