DIAGNOSIS That's coil whine. It's not a sign of a defect that will cause a failure, but it's an annoying acoustic artifact from electrical components (like inductors or capacitors) on the internal circuit board vibrating at a high frequency.
FIX Unfortunately, there's no software setting to fix this. The first step is to try a different power outlet on a different circuit in your home. Sometimes electrical noise or the quality of power from a specific outlet can exacerbate the sound. If that doesn't change it, the only reliable fix is a hardware replacement. Contact Linksys support. Describe the audible coil whine. Since this is a known but not universal issue with some electronics, they will often authorize a replacement under warranty if the sound is pronounced.
IF THAT DIDN'T WORK If a replacement unit also has the whine (though likely fainter), and it's driving you nuts, your last resort is to move the node to a location where you can't hear it, like a closet, attic space, or cabinet. Just ensure it still has decent airflow and isn't completely sealed in, as it needs to vent heat.
HEADS UP Don't try to open the node to fix this yourself. You'll void the warranty and risk damaging sensitive components. The sound may also be more noticeable in a very quiet room at night. Some people's hearing is more sensitive to that specific frequency than others.