Is heart murmur with mild leak dangerous?

Probably not: It depends on which valve is leaking and how severe the leakage is. But it probably is not dangerous, especially if you feel ok.

Could be normal?: Most doctors and nurses do a bad job at explaining what a "murmur" is... A murmur is simply an extra sound we hear when listening to the heart. Instead of just hearing "lub-dub" we hear an extra swoosh... Same type of sound we hear when we kink a garden hose with water running through it. If the sound "murmur" is coming from a leak in a valve, it really depends which valve is leaking.