DIAGNOSIS That's coil whine. It's not a sign of failure, just an annoyance. It happens due to electrical components (inductors/coils) vibrating at a high frequency under certain loads. It's common in electronics and doesn't affect performance or lifespan.
FIX First, identify which unit it is and if it changes. 1) Listen to each unit individually by putting your ear close. 2) Note if the noise gets louder when the network is under heavy load (like a big download or multiple 4K streams). Often, there's no true 'fix' from TP-Link, but you can try to mitigate it. 3) Make sure the unit is on a stable, hard surface and the power cable is seated firmly. Sometimes vibration resonance on a shelf can amplify it. 4) Try plugging it into a different outlet, preferably on a different circuit, to see if 'cleaner' power reduces it.
IF THAT DIDN'T WORK If the noise is very loud and constant, and the unit is new, it could be a defective power supply component. Your option is to contact TP-Link support about a possible replacement under warranty. Describe it as a loud, constant electronic whining noise from the power circuitry.
HEADS UP Don't open the unit. There are no user-serviceable parts inside, and you'll void the warranty. The noise is almost always harmless, just try to place that particular unit in a less quiet room if it bothers you.