rale vs rales

rale

noun
  • An abnormal clicking, rattling or crackling sound, made by one or both lungs and heard with a stethoscope, caused by the popping open of airways collapsed by fluid or exudate, or sometimes by pulmonary edema. 

rales

noun
  • Abnormal clicking, rattling or crackling sound heard from the lungs, often audible only with a stethoscope. 

How often have the words rale and rales occurred in a corpus of books? (source: Google Ngram Viewer )