Your heater should operate silently, but it’s normal to hear a clicking sound when the heat turns on and off. Oil heaters can also produce some crackling noises as the oil inside moves when it heats up and cools down.
If you hear knocking or banging sounds, these are likely coming from the heating element, and they are not dangerous. Panel heaters generally have an aluminium element that heats up. Aluminium expands when it gets hot and contracts when it cools down. To accommodate this, the aluminium element is mounted on a track inside the heater, allowing it to expand without issues.
The knocking or banging noise occurs when the heating element doesn’t slide properly on its tracks. You can think of it like a gear that doesn’t engage properly with another gear. While not harmful, it can be an annoying sound.
If your heater makes these noises, I recommend checking if it’s still under warranty. You can contact the store where you purchased it. If they can’t assist you, reach out to Mill customer service for further support.