Almost any switch if moved slowly enough will make that noise. Its arcing between the contacts inside the switch. In some you can even see the spark in the dark. For a properly working switch the only way it usually happens is if you intentionaly hold the switch in the middle.
This arcing will eventually damage the contacts of the switch, causing it to fail prematurely. If the switch got stuck in that position long enough the heat from the arcing could potentially cause a fire. Usually the switch will simply fail without causing a fire, but it is a possibility.
If the switches feel lose or don't quickly snap between on and off positions, they need to be replaced.
If you are just intentionally moving the switch between positions very slowly - stop doing that! The switches may still need replacing from the damage the arcing has already caused.