Over time, lime builds up inside your kettle.
To descale your kettle, fill it with water up to the MAX level and boil. After your kettle is switched off, unplug it. Add two cups of white vinegar or one tablespoon of lemon salt into the boiled water. Keep the solution in the kettle for a few hours. Then pour it out and rinse the inside of your appliance thoroughly. Fill the kettle with clean water and boil it. Empty the kettle and rinse it with water again.
You can repeat these steps if there’s still some lime build up. You can also use a commercial kettle descaler. Make sure you follow the instructions on the package.