Shuttering sound at water main?

[Brianwrest]

We just purchased a house with a Toro-based system. I went through the reactivation sequence provided to me (via Flier's in Grand Rapids, MI) and when the zones initially will start-up (one-by-one), I hear a loud shuttering sound at the main.

What might cause this?

 

[Gary Swart]

May be air being expelled from the pipe in the drained zone?