There are several possible causes of duct noises. Have you ever had fun rattling a sheet of metal to make “thunder”? Ductwork can settle after some years and twist to a point to make noise as it expands and contracts; this is called “oil-canning” which is a popular description of duct noise. As the blower turns on and off and the metal changes temperature the sound occurs.
Sometimes holes are left open so squirrels and other forms of life run through the convenient pathway to distant places. People are often seen to take route as popularized in Hollywood film. Rushing air (and flames during fires) makes noise as it slides past corners and sharp edges.
Ductwork is a punishment for not being able to afford hot water heat and ductless air conditioners. It is very popular in low-bid tract homes in low-bid communities in low-bid states.