This can be from air trapping in a previously healed lung infection that is causing the noise / murmur when you breathe.
This can also be a heart murmur, but your doctor would have recognized it if it were heart related.
If you feel this murmur and have any breathing difficulty, then you should see a doctor and have an X ray of your Chest done, that would look for any organic cause that is giving rise to this murmur.
Kind Regards, Dr.Joshi
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