I opened up my backflow preventer and turned my sprinkler system on for the first time since the summer and noticed an awful water hammer like sound from my PEX pipes in the attic when it switched to a new zone. It only last for a few seconds during each zone switch (7 zones), but it was loud and violent sounding enough to wake up napping kids.

I've never had any issues with the system before, so what could be causing this issue all of a sudden? Also, I thought an irrigation system ran independent of your home plumbing. So other than the tie in to the main water line, why would an irrigation system affect plumbing in the home?

Side note - I was told by a neighbor that the guy who installed the irrigation system did it for several neighbors as a package deal and that he likely wasn't licensed. So I'm questioning the design and installation of my system more than ever now.

TIA