To hear crackles, the animal must be taking sufficiently deep breaths to inflate the lung. Consequently, they are usually loudest at the end of inspiration. Crackles can be either coarse or fine.Fine crackles are usually only heard at the very end of inspiration and are probably generated by the opening of collapsed small airways. Coarse crackles are usually associated with parenchymal disease but occasionally can be due to airway disease.Nevertheless, if you hear coarse crackles, it is most likely that the animal has a fluid build up of some sort in its lungs. But by auscultation you can’t tell what that fluid is. It could be blood, exudate from pneumonia, hydrostatic oedema from left heart failure or fluid overload, neoplasia related fluid, or neurogenic pulmonary oedema.These radiographs are from a dog with probable aspiration pneumonia. You can see that the middle lung lobe has a quite dense alveolar pattern and that is where you would hear harsh lung sounds or crackles or both.