Murmurs are extra sounds heard over the heart and are
associated with eddies and flow turbulence. Many murmurs are
organic indicating valvular defects or other abnormalities within
the heart. Some systolic murmurs are functional (they have no
organic cause) and are of no consequence. Differentiating between
organic and functional murmurs is often difficult by auscultation
alone.
Murmurs are classified according to their location; they are usually most obvious over the area with the defect. The loudness of systolic murmurs are graded on a scale of 1-6 (faintest to loudest). It should be also be noted whether a murmur is associated with mid or late systole or diastole, or if it is continuous. Diastolic murmurs are graded on a scale of 1-4.
Murmurs may be considered high pitched (with a frequency of 300 Hertz or more), medium pitched (100-250 Hertz) or they may be rumbling or low pitched (60-100 Hertz). The sounds may be rough or harsh. Some murmurs may be felt with the flat of the hand against the chest or with the stethoscope barely touching the chest if they are very high grade such as 5 or 6.
Systolic murmurs are usually created by valves which 
leak
during ventricular contraction (early, mid or late systole) or by
stenosis and interference of blood flow. Diastolic murmurs
usually occur during ventricular relaxation and filling. If a
murmur is detected during systole, it 
may
indicate mitral or
tricuspid valve insufficiency. If detected during diastole it may
indicate aortic or pulmonary valve insufficiency.
The scarring of valve leaflets often prevents them from opening properly and also narrows the valve area (stenosis). With mitral valve stenosis, flow from the left atrium to the left ventricle is impeded during diastole. Stenosis of the aortic or pulmonary valve impedes flow to the aorta or pulmonary artery causing a systolic murmur. A murmur produced by a valvular lesion is usually heard loudest over the involved valve area.
Murmurs may also be produced when an abnormal amount of blood passes through an orifice such as in anemia, hyperthyroidism or after extreme exercise. Murmurs also occur due to abnormal communications between vessels and heart chambers. The vibrations produced may not only create murmurs but may be felt as "thrills". For example, in aortic stenosis, a thrill may often be felt over the precordium with its maximum intensity at the aortic area.