Noise in the gas pipeline (Like running water)

I have just had installed a 12000btu carrier Split Unit. The distance between the indoor and outdoor units is about 22 meters (70 yard). The cooling is fine but when turned on a hissing sound is very noticeable from the pipes as though you have running water.

It is very annoying specially that the majority of the pipeline runs indoors.

The technician says this is normal due to the length of the pipeline.

Is this true or could it be due to another factor e.g. gas/oil in the system?

Appreciate your feedback