Ducks “quack” (graznido) or make the sound “quack-quack (cuac cuac). This is the noun.
The word “quack” (graznar) is also a verb. An example: the duck is quacking (graznando).
There are other uses of the word. There are quack doctors (medicuchos o curanderos), and such doctors have “quack remedies or medicines” (medicinas curanderas). Such doctors practice “quackery” (curanderismo).