Sounds like you have a grade 1 murmur. That's one that you have a good chance of missing if you have been to a loud rock concert the night before, or you are using a cheapo NHS stethoscope because somebody has pinched your fancy cardiology one. Often first picked up by students who don't have a clue what they are listening to but have good hearing, though these walkmans are taking a toll on that.
So it could be considered bad luck they found it in the first place.
An ECG isn't a particulary useful investigation when digging into a grade 1 heart murmur in a fit and healthy youngster. ECG looks at the electrics, heart murmur is down to the plumbing. An echocardiogram uses sound waves to look at the blood flow in the heart, and the fancy electronics can show turbulence in the flow which causes the noise. Sounds like you are on course to shell out a load of money for the test.
This comes up quite a bit with fit and healthy young people doing class 1 at Gatwick, and it appears the CAA have a policy to get a second opinion. Doesn't happen in other parts of the world, and planes aren't falling out of the sky as a result.