Heart murmurs are abnormal heart sounds that can be heard when listening to the heart with a stethoscope. They may be caused by turbulent blood flow through a heart valve or by a structural problem in the heart. Murmurs can range from soft and barely audible to loud and easily heard. They can develop suddenly or gradually and may be described as blowing, swishing, or whooshing. Murmurs can be classified as systolic, diastolic, or continuous, depending on when they occur in the cardiac cycle.