Why Is Main Water Supply Line Making Whooshing Sounds?

Why do we have a pulsing/whooshing sound in the main supply line when water is flowing (faucet open, toilet filling), but the sound goes away when valve closes?

These noises often occur when the pipes are beginning to develop build-up on the inside. Often galvanized pipe and copper pipe will have these types of noises, because of the harder turns and the reverberation that occurs because the pipe is metal. Galvanized pipe often develops calcium and mineral deposits on the inside, which causes the water to swirl and create noisy water pipes.