Blood pressure measurement
The sound in question is called the Korotkoff sound:


  If the pressure is dropped to a level equal to that of the patient's
  systolic blood pressure, the first Korotkoff sound will be heard. As
  the pressure in the cuff is the same as the pressure produced by the
  heart, some blood will be able to pass through the upper arm when the
  pressure in the artery rises during systole. This blood flows in
  spurts as the pressure in the artery rises above the pressure in the
  cuff and then drops back down beyond the cuffed region, resulting in
  turbulence that produces an audible sound.