DIAGNOSIS That's coil whine from the power supply or the control board components. It's a high-frequency vibration in the electronic components, not a mechanical fault. Some units just have it, and it can be more noticeable in a quiet room.
FIX First, ensure it's on a level surface. Vibration can amplify the sound. If it's new, let it run for 24-48 hours. Sometimes components 'settle in' and the noise diminishes. If it's unbearable, contact NewAir support—this can be grounds for a replacement under warranty if it's exceptionally loud.
IF THAT DIDN'T WORK The noise might be coming from the thermoelectric module itself (a different kind of whine under load). If it changes pitch when you change the temperature setting, it's likely the module. There's no user fix for this.
HEADS UP This noise is often more noticeable when the unit is first cooling down from room temp. Once it reaches its target temperature and the cooling cycles on and off, the whine may only happen intermittently, which can be even more annoying.