What could causes a heart murmur in an otherwise healthy 17yr. Old?

Could be normal. Thin young healthy teens can have turbulent blood flow heard as a murmur. Heart valve abnormalities can lead to murmurs, but are rare. Sometimes, if a teen is anemic, the increased heart rate from that will cause the murmur. Have the doctor listen carefully, and an echocardiagram may be needed. This is a painless, not invasive test which uses sound waves to look at the heart structure.
Murmurs in youth. Healthy young people, especially athletic ones, often have benign heart murmurs. However, this is something that must be evaluated by a doc. A doc needs to hear the murmur, note where it is coming from and possibly do an ultrasound exam of their heart. Especially if this is a new murmur, you need to see your doc.

Related Questions

What causes a heart murmur? I was diagnosed with a heart murmur as a child. Should I be worried as an adult? What causes heart murmurs and are there any symptoms to look for?

A. A murmur is simply a sound coming from the heart. It could mean nothing or be very serious and require treatment and follow -up. See a cardiologist and get an echocadiogram with doppler of your heart. This will tell the story. And you will know exactly what you need to do if anything. Don't wait!
Heart. Heart murmur is a sound heard with a stethoscope that generally represents turbulence from within the heart, usually turbulence from the blood passing over the valves and sometimes other reasons such as tortuous vessels. Murmurs can be innocent or physiologic particularly in young persons because their hearts are super healthy and contract with more force than an adult heart creating a physiologic turbulence across the valves. This is why childhood murmurs do not always equate to heart disease. However there are a number of childhood heart problems, usually congenital or due to rheumatic fever, that do represent cardiac pathology. Get your childhood records or speak to the childhood doctor and see a current physician to see if you have a heart murmur at present and have them review your childhood health records with you and consider a current test of your heart particularly an echocardiogram.
Many causes. A murmur simply means that someone listening can hear blood flow. A murmur is not a thing in itself. Many people with normal hearts have murmurs. But on the other hand, murmurs can also be caused by abnormalities in the heart (e.g. holes, valves not working appropriately). Please discuss your specific case with your doctor; further testing may or may not be needed.

Could a heart murmur be a cause for concern?

Depends. There are many different types of murmurs, not all are a problem. A good physical may be all that is needed. Other testing such as echocardiograhy can be very helpful. See your doctor.