We have an older (about 20 years) Trane natural gas furnace in our house. We haven't had any trouble with it. When we had it serviced for the winter, however, the tech suggested that we change out our gas stop and put one on that is easier to turn off in an emergency. He changed out the gas stop and installed a flex line leading from the gas stop to the furnace (before we had a metal pipe).

Now when we use it we can hear a whistling noise that seems to be located around the flex line. It echoes through the whole house, we can hear it coming out of the registers. I have seen some references to flex lines that are too small making this type of noise, or people have suggested that it could be the shape that the line is bent into making the gas whistle through it.

My question is, does anyone think that this could be it? Or maybe it is a more serious problem?

(It makes the sound intermittently as well, sometimes it does it and sometimes it doesn't.)