Why does my gas pipe sing?

[Vatmax]

I did some re-piping recently to my gas line in the house. I've checked for leaks several times and everything looks good. No smells either.

The gas line seems to be making noises. It was hard to locate at first then I realized that it was probably the entire pipe resonating. Does anyone know what that could be? Is there air trapped? Maybe the shut off valve on the meter has a problem or something?

Any help is appreciated.

Thanks!