An added sound heard when listening with a stethoscope, distinct from heart sounds or other clicks or snaps.

Can indicate a structural abnormality.  But can be heard in normal hearts too, esp kids.

Still’s murmur

Or “innocent” murmur.  Characteristic vibratory, low pitched, crescendo-decrescendo sound – “like an Aeolian harp” – loudest along left sternal border.  Never louder than grade 3.  Typically gets quieter when child sits or stands up (reduced venous return?) – you would not expect a murmur caused by a structural abnormality to change much.

Present at any age. Usually goes away by adolescence.

Venous hum

Another innocent one, a rumble heard in the upper chest, disappears when lying down, or when neck turned or neck veins occluded gently.

Pulmonary flow murmur

Pulmonary valve closest to anterior chest wall, which might explain why you sometimes hear this.  Might be confused with pulmonary stenosis or subaortic membrane.