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.
The results of a voltmeter vary depending on the type of voltmeter used. For a more accurate result, be sure to use a voltmeter with more than 1 megaohm input resistance for alkaline buttons batteries or a 10+ megaohm for lithium batteries.
The capacity of batteries is indicated as XXXX mAh (milliampere/hour). If you insert this battery into an appliance which consumes 100 milliampere current continuously, the operating time of the appliance will be around 20 hours mathematically.