Switching mode power supplies (SMPS/PSU), also called “LED drivers”, contain coils, such as transformers or inductors that use electromagnetism to convert AC mains power into low voltage DC.
The varying magnetic fields generated by those components can cause them to physically vibrate at a high frequency, resulting in a high-pitched noise. Additionally, because pulse width modulation (PWM) - used for voltage regulation - will be at a low duty cycle, a "spikey" output might cause further vibration in the coils and as a result an audible noise is generated, which becomes noticeable on the chassis mount of the SMPS.
Also, we would like to mention that most chassis mount LED drivers, which put out more than 150W in power, come with a built-in fan, which can also cause a lot of noise (an example being if installed in a bedroom environment).
Because enclosed "waterproof" SMPS are encapsulated (special HV resin), they are “noise free” and no fan is required. This is because they are designed to cool down through free air convection. As such, it makes them a better choice in some cases, depending of course on what the individual installation needs, budget and power requirements are.