popping noise from power window?

When a power window is being rolled down and it is slow and makes a popping sound, what could the cause of this problem be? I have an idea about the slowness but i don't know about the popping sound.

By the way its not my car that is doing this it is a friends mazda.

take apart the door from the inside and you'll see a motor that coils the wire that pulls the window up or down. When it pops, it's the wire coiling and then sliding out the side instead of winding properly. You might hear the same sound when your power antenna is malfunctioning.

thanks