A heart murmur is the flow of blood over a heart valve loud enough to be heard with a stethoscope. They can be caused by conditions that are actually outside the heart, or by abnormalities in the heart itself.
Murmurs are most commonly caused by leaking or narrow heart valves, but can also be caused when an abnormal blood vessel is adjacent to the heart.
Patent ductus arteriosus, Turner syndrome, Eisenmenger's syndrome, and patent foramen ovale can all cause heart murmurs.