DIAGNOSIS That's coil whine—a high-frequency buzzing or clicking from the internal power electronics. It's not dangerous, but it can be audible in a quiet room. It's more common with the power adapter brick than the satellite itself, but it can come from either.
FIX First, identify the source. Unplug the satellite's power adapter from the wall and listen. If the noise stops, it's the adapter. If it continues (using battery backup for a moment), it's the satellite unit. For a noisy adapter, try plugging it into a different outlet, preferably on a different circuit. Sometimes poor power quality makes it worse. You can also wrap the adapter in a bit of felt or place it on a soft surface to dampen any vibration.
IF THAT DIDN'T WORK If the noise is definitely from the satellite body and it's persistent, it's likely a hardware fault. Contact Netgear support. They are generally aware of coil whine issues on some units and may offer a replacement under warranty if the noise is deemed excessive.
HEADS UP Don't cover the vents of the satellite or adapter to try to muffle the sound. These units need airflow to stay cool. Blocking vents will cause them to overheat and throttle performance or fail.