Question
I live in st louis mo. When my system is turned off, main water supply pipe is banging back and forth.At same time the manifold is releasing water in a pulse.This does not stop untill water supply line valve is turned off.System is 12 yr old.Problem has just started.

Answer
Hey dirtbag,
 I live in Saint Louis, too.
 Sounds like an old valve is closing too quickly, assuming you haven't done any recent changes to the system - like adding some heads to a zone or putting bigger nozzles in heads. Then by manifold I assume you mean some kind of backflow preventer starts dumping from the relief port.
If system is Toro, cracked seats in the valve will do this. And 1" Toro valves always get cracked seats over the years. Since your local get back if you want me to come by and look at it. No Charge to look. Jim