Yes, it's normal. If you look along the outer part of the sash, you see a fine, lets call it rope. You'll see it go up into a hole in the frame. There is a spring loaded pully in there that when you raise the window, recoils the rope onto the pully. This keeps the window up. When you lower the window, the spring gets tighter and makes the twang noise. With the window down, if you look at the sash and pull on the rope, you'll see it is taut and recoils back up in there when you let go. This takes the place of the old time sash weights that used to do the same job. You'll see this on each side of the window where the window tracks up along the frame.

Look at the window sticker to make sure the windows are the 400 "Woodwright" model made by the Andersen Corporation for new construction and not a replacement window or a knock-off.

My Andersen windows have the "twang" also.