It is caused by abnormal blood flow in the heart. (catdepot.org)
A heart murmur can be caused by abnormal blood flow within the heart, usually involving the heart valves. (healthypets.mercola.com)
Diseases affecting the valves result in abnormal blood flow in the heart (valve disease is a common cause of murmurs), which places strain on the heart as it tries to compensate for abnormal flow through thickening of the muscles and dilation of the chambers. (morrisanimalfoundation.org)