LED Drivers "High-pitched Noise"

Switching mode power supplies (SMPS/PSU) also called “LED drivers” contains coils, such as transformers or inductors that using electromagnetism convert AC mains power to low voltage DC.

The varying magnetic fields generated by those components can cause them to physically vibrate at high frequency, resulting in a high-pitched noise. Additionally because of pulse width modulation (PWM) used for voltage regulation will be at a low duty cycle a "spikey" output might cause further vibration in coils and as a result an audible noise is generated that generally on chassis mount SMPS will be noticeable. 

Also I would like to mention that most chassis mount LED drivers more than 150W in power will come with built-in fan and that will also cause a lot noise (as an example if installed in a bedroom environment).

Because enclosed "waterproof" SMPS are encapsulated (special HV resin) they are “noise free” and also no fan needed because are designed to cool down through free air convection. This makes them a better choice in some cases depending of course at the individual installation requirements, budget and power requirements.