Base Left: Second Sound (S2)
The closure of the aortic and pulmonic valves contribute to the production of the second heart sound or S2.
The aortic (A2) component of S2 is the loudest of the two components and is discernible at all the auscultation sites. However, it is best heard at base right, with the diaphragm of the chestpiece firmly pressed.
The pulmonic (P2) component of S2 is the softer of the two components and may only be audible at base left, with the diaphragm of the chestpiece firmly pressed.
The sound you heard is an S2.