These murmurs are due to blood flow from a high pressure chamber or vessel to a lower pressure system.
Patent ductus arteriosus. PDA is an abnormal connection between the aorta and the pulmonary artery, which normally should be closed in infancy. Since aortic pressure is higher than pulmonary pressure, a continuous murmur occurs. This murmur is often described as a machinery murmur, or Gibson's murmur. This is named for George Alexander Gibson, who characterized it in 1898.