You'll find knockers in mines.  They knock on the walls and sound like the miners mining there. Rumor has it that they mine at night when the miners are gone from the mines, but no one knows exactly what they are looking for. These fellows are good because they try to warn miners of dangers by pounding on the walls. Knockers don't like whistling or swearing and will cause small stones to fall from the ceilings when they are troubled in this way. Knockers can also be found in wells, basements, caves and even in basements.
* photo are courtesy of Arthur Spiderwick's Field Guide