The Knockers, known also by several different names including knacker, coblynau, bucca, polterduk, karkorios, rübezahl, or pustecki, are larger and stronger breed of goblin, even though they are rarer. They inhabit various adits, grooves, precipices, chasms, dark cavities, burrows, delves, grottos, caverns and rocky heaths. Their existence in the area is always evidence for the presence of wealth hidden in the ground, such as ores, minerals, salt or petroleum. Considered as the malicious rogues and pests, and the real scourge for miners who they tantalize and haunts by knocking the rock, steal or destroy sidewalks, mining equipment and all possessions, sometimes even attack but stop at a few bruises. However, miners can get at them by food and alcohol.
"The Knocker, likewise called a knacker, coblynau, bucca, polterduk, karkorios, rübezahl, or pustecki, is a form of kobold, which, nonetheless, the K. considerably surpasses in magnitude and strength. The K. as a rule also wears a great beard, which kobolds habitually do not. The K. dwells in adits, vertical shafts, spoil heaps, precipices, tenebrous hollows, inside rocks, in diverse grottos, caves and stone wildernesses. Wherever it dwells, natural riches such as metal, ore, carbon, salt or petroleum are surely buried in the earth. Thus, one may often encounter a K. in mines, particularly abandoned ones, although it is also likely to appear in active ones. It is a vicious scourge and pest, a curse and veritable divine retribution for miners and quarrymen, whom the vexatious K. leads astray. By knocking on the rock it beguiles and frightens, obstructs galleries, steals and spoils mining equipment and all kinds of belongings, and is also inclined to strike one on the head [from] a place of concealment. But it may be bribed, to curb its mischief-making, by placing in a dark gallery or shaft some bread and butter, a smoked cheese, or a flitch of smoked gammon; but best of all is a demijohn of alcohol, since the K. is extremely greedy for such." - pg. 227, The Tower of the Swallow (US edition)