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 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.
Panasonic hearing aid batteries, or zinc air batteries, react with air. As soon as you pull off the seal, the battery will be able to release energy. Be sure to let the battery breathe for a few minutes before inserting it into your device.