[Thanks to NETGEAR for this explanation!]
If you can hear a faint hissing or buzzing sound from your router, wireless Access Point or other piece of networking gear, there’s no cause for alarm! The noise is from a coil in the DC-to-DC converter in the product’s power supply.
In the past, these circuits were designed to run at a fixed frequency outside the audio range (like 40 to 100 kHz) so that you wouldn’t hear them. But newer (and cheaper) designs let them free run, i.e, change oscillation frequency, occasionally generating noise that you can hear.
Apparently, users’ ears are sharper than the producers of networking gear thought! So they’re working on a way to make sure the power supply switching frequency stays up in the non-audible range for future designs. At any rate, it’s nothing to be concerned about in terms of product reliability or safety.