A murmur is an abnormal sound heard in the heart or major vessels. There are many different causes. Murmurs may occur at birth when they may be caused by major anatomical abnormalities (dont worry guys, you would know if you had this!) or by persistance of the foetal circulation.
Other murmurs can start in later life and usually signify a leaking or narrowed heart valve.
BUT they dont come and go - the only exception is that some murmurs are louder on exercise etc but normally it is simply one doctor hears it and another doesnt
Because these are all anatomical problems the test is an ultrasound which produces a picture of the problem - simple as that
Many murmurs are 'benign' which means they do not signify significant limitations of the pumping action of the heart. Apart from needing antibiotics with operations you will live to die of other causes the same as the rest of us.
So have an ultrasound, get a diagnosis, and then your AME can decide whether you can fly or not.