I have a sprinkler system with 3 legs. One leg has stared pulsating the pipes in the house when it is on. The other two legs do not. I have a gate valve on this leg to reduce pressure but the pipes pulsate no matter what the pressure is. This system has operated for many years properly. This is the 1st year this has happened and no changes have been made. Any help is appreciated.

Gate valve doesn't reduce pressure. It can only reduce volume. In other words, you are choking water supply to sprinklers.

Also, it is most likely the culprit. I guess the vibration is caused by yoke inside the gate valve. Try to open the valve all the way. See if it continues.

If you want to reduce pressure than use Pressure Regulator.

Back to you... Milo

I didn't know that a gate valve wouldn't reduce the pressure so I will add a regulator to this leg as this leg is a low flow system. However, I have opened the gate valve all the way and closed it gradually and the only change was to the frequency of the pulsations. I also connected a hose to a spigot on that leg and opened that valve all the way and no change there either.

Without seeing the job, I think the gate valve is bad... Replace it with ball-type valve...