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There could be several explanations –
The whole might not have healed completely – a small residue might be remaining.
There may be other causes that can be highlighted after the closure of the hole.
There may be other causes acquired after birth.
So on so forth.
You need not worry unnecessarily.
The murmur is most likely haemo-dynamically – clinically – NOT significant.
You have no other symptom...
Smaller the defect, louder the murmur!
It is simple to evaluate.
See your doctor –
will clarify most of them.
It is cheap and non-invasive.
Why then sit down and worry?
Wishing speedy recovery