There are several reasons behind the cause of an abnormal heart murmur. Many babies are born with a diseased valve in their heart causing a murmur. Septal defects and cardiac shunts are other causes behind abnormal heart murmurs which are usually holes in the heart and are found mainly in babies. Rheumatic fevers are dangerous and can often be a reason behind heart murmurs. These fevers can directly affect the valves and cause abnormal heart murmurs. Mitral valve prolapse in adults can often suffer from abnormal murmurs. Since the valve balloons out, the mitral valve is unable to close properly. The backward flow of blood through the valve causes abnormal heart murmurs. This can be a dangerous issue. Endocarditis is serious heart disease and can cause abnormal heart murmurs. The infection present in the heart lining is caused due to endocarditis. The bacteria present in the body might settle in the heart causing endocarditis. Valve calcification is a major cause of heart murmurs. The calcium deposits present in the aortic valve develop the disease. The opening of the aortic valve becomes narrow making it difficult for blood flow. Other than these, various hidden diseases might lead to a heart murmur.