I seriously think most humans all have some sort of heart murmur. What is feels like if there is nothing seriously going on is exactly that...nothing. I know that when it becomes a little more active you may feel it in your neck and chest for a few seconds. Its great that the docs are having to search for it because that means its not too serious otherwise they might hear a pronouned clicking in their steth. i would relax if you aren't having any problems with it and no generally as you get older it doesn't get worse. It just may be the physical make up of your heart. Our hearts are kind of like our fingerprints...everyones are different. I wouldn't stress it....
There are different grades of heart murmur. Yours must be very mild as is mine. If it was more severe, they would have sent you to a cardiologist somewhere along the line to have your heart checked (valves, heart size, etc.). From what you're telling me, your murmur is just something they can hear but not of great concern. The next time you have to see a doctor, you can ask how your heart murmur is doing. They'll check it to make sure it's still in the "mild" range.
Are you having any symptoms that might be heart related - pain, short of breath, extreme fatigue, edema, turning blue? If your health status changes, then you should obviously see your doctor.