A heart murmur is just a noise that the blood makes as it flows through the heart. It's like the noise water makes when it flows through a hose.
Many healthy people have heart murmurs.
About 80 percent of all children have a heart murmur at some time. Most heart murmurs don't mean there is anything wrong. A normal murmur is just a noise caused by blood flowing through a normal heart. A normal murmur can get louder when the blood flows faster through the heart. For example, when the body's temperature goes up, the blood flows faster.