Cleaning Your Humidifier
Humidifiers should be cleaned every year because they build up scale, rust, and hard water marks as they operate. Your humidifier may be disassembled for cleaning but each system has a proper technique.

Pads and Filters
When your humidifier's pad or filter gets covered with hard water scale, you likely won't be able to get it perfectly clean again. It's best to just replace it each year.
Humidifier Float
In a reservoir-type humidifier, the float assembly may get stuck while sitting unused all summer. To avoid this problem, remove the float assembly after each heating season and thoroughly clean and inspect it. For cleaning, try using vinegar.
Water reservoir and bacteria control
Over time, the water reservoir on reservoir-type humidifiers gets encrusted with calcium and other minerals. It may also begin to harbor bacteria. A humidifier cleaner and anti-bacterial solution can be used to clean the reservoir completely at the start of the heating season, periodically throughout, and again at the end of the heating season.
Humidifier Noise
While humidifiers run, they either add water to the filter pad or reservoir, or they spin their cylindrical pad through the reservoir. The water being allowed to run into the flow-through type can be noisy, as can the motor that spins the pad in a reservoir. There's usually no remedy for this noise. If it's very noticeable or bothersome, a qualified heating and cooling contractor may be able to help.
-- Tips courtesy of HomeAdvisor.com