I have a heart murmur with not even 1 symptom. is something wrong or is it nothing?

Probably innocent: Significant (pathologic) murmur is associated with one or more of the following: cardiac symptoms, cyanosis, systolic murmur of 3/6 or higher degree, diastolic murmur, presence of click, or abnormal pulse. Well experienced doctor can easily distinguish one from another. Not too infrequently, children with innocent murmur are referred to cardiologist 'to be sure.'

Innocent v bad: Murmur is a label meaning a sound is heard other than the lub/dup of the normal heart beat. If I listen to a person often enough over their life I will hear a murmur at some point. It can be innocent (a murmur with no importance) or bad news. In kids murmurs can mean malformations that require surgery to live. In adults it can signal changes in a normal heart. Your doc can explain if you ask.