A heart murmur is usually caused by a mitral valve prolapse. This is when the valve is slightly deformed causing it to not seal completely. It can be caused by various things including infections, or it can be from birth. In most cases there are no symptoms and no reason to worry about it. Most doctors even consider a mild prolapse to be normal. Unless you have other health problems, or your doctor says otherwise, exercise should be fine. Good aerobic exercise is great for heart health, and your overall health. Be sure to check with your doctor before starting any exercise program, but the murmur shouldn t really make a bit of difference. Take care.