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A murmur is an abnormal sound heard on auscultation of heart sounds
It is a subjective finding depending on the observer but being skinny individuals will not cause it.
Ther are many causes of murmurs
some benign physiologicalnot so worrisome some pathological due to heart or systemic disease.
It is best to get Echo and ECG done if your doctor has suggested.
The report needn't be something to be scared of necessarily as I mentioned above ,sometimes even thyroid
etc which are quite treatable can cause murmurs.
So don't worry.but get yourselves thoroughly investigated and treated by a physician