Diagnosis
A faint, rhythmic ticking or buzzing from the power adapter or the router itself is usually coil whine—a high-frequency vibration from electrical components. It's not necessarily a sign of failure, but it can be annoying.
How to Fix It
First, isolate the sound. Unplug the power brick from the wall and listen to it. If the noise is coming from the brick, that's your culprit. Try plugging it into a different wall outlet on a different circuit, as dirty power can exacerbate coil whine.
If That Didn't Work
If the sound is definitely coming from the router's main unit, it's less common but still possible. Ensure the router is on a stable, flat surface and has plenty of ventilation. Overheating can sometimes make electronic noises more pronounced.
Heads Up
Coil whine, while irritating, doesn't typically affect performance or longevity. However, if the noise is a loud, irregular clicking or grinding, or if the router is also overheating or rebooting, then you should contact ASUS support as the internal power circuitry might be faulty.