Why is my transformer making a humming noise?

Low voltage transformer cores are built from thin steel laminations stuck together. The cycling on and off over time may have loosened the laminations, creating a slight vibration. This is typical of units that have not been encapsulated with epoxy during the manufacturing process. It does not mean that your power supply will fail.

How do I size a transformer for my lighting installation?
How many LEDs can the electronic transformer handle?