Why does my electric fan crackle sometimes?

I have a big plastic fan that I use to cool off when my mom refuses to turn on the AC. It plugs in to a wall and emits a lot of air. Sometimes when I use it it starts making crackling sounds and it kind of freaks me out. My mom uses a smaller fan that doesn't do anything like this. Could the coagulated dust on it have anything to do with the crackling sounds it makes? Any electronic experts have any idea what it might be and if it's dangerous?