LLSB: Third Heart Sound (S3)
The third heart sound, or S3, is a low frequency sound which occurs in early diastole during the time of rapid ventricular filling. It occurs about .14-.16 seconds after the second sound. The S3 reflects decreased ventricular compliance or increased ventricular diastolic volume.
The normal S3 can be heard in children and young adults (not past the age of 30), whereas the abnormal S3 is heard in individuals with coronary artery disease, cardiomyopathies, incompetent valves, VSD, and PDA.
The S3 of left ventricular origin is best heard at the apex, whereas the S3 of right ventricular origin is best heard at the LLSB. S3 sounds are often associated with sharp outward precordial movements which frequently can be seen or felt.
The sound you heard is an S3 manifesting congestive heart failure.