This is a vibration noise which occurs when electric current flows through a transformer or a winding wire inside a component part. However, the charger may still be used without any concern as there is no need to worry from a safety point of view.
Batteries in smart keys usually last between 1 to 2 years. Unusually short battery life may be caused by electromagnetic interference. Avoid keeping your smart key near other electronic devices such as phones, laptops and other screens.