To give some background, we recently had an plumber out as we had water hammer whenever our sprinklers would turn off (we had no issue with them turning them on, just the hammer when they would turn off). The plumber found two issues: 1) the pipes weren't secure, so were moving around quite a bit with the force of the water turning off, and 2) our expansion tank needed to be replaced. The plumber secured the pipes and replaced the expansion tank. All seemed to be quiet and working properly!
Fast forward to 3 days later. Our sprinklers came on and everything seemed to be working well...until zone 2 came on. When zone 2 turns on, there was a loud bang and then a loud thumping for a about 7 seconds or so (seemed like forever) until the system filled and the water flowing out of the sprinklers. No problems with any of the other zones. When I ran the system again about an hour later, no bang or thumping and all seemed to be working. It's been 3 weeks now, and every time zone 2 comes on, I get the same issue. It is the worst whenever zone 2 comes on for the first time after not using it for a while. We have 7 zones total, and this only happens on zone 2 - the other zones are smooth (in fact, you can barely tell they are on). We had the plumber come back out and he thinks it is the valve in zone 2.
I recently found this thread:
Thumping/Knocking Noise in Pipes for a Few Seconds when Valve Opens/Zone Comes on and noted post #13 as a potential solution. We do have a PRV between the incoming line/sprinkler system and the rest of the house, however, when they checked the pressure to the rest of the house, they said everything (regarding pressure) was fine.
Since this is only occurring on zone 2, my guess it that it might be a combination of our sprinkler system and the solution found in the link above. It is possible that zone 2 is somehow losing more pressure than the rest of the zones in-between cycles causing this issue? The pressure fix makes sense to me, but if that was the case, I would think there's be a problem on all of the zones. My thought is maybe some kind of leak that is causing the pressure drop in zone 2, but I'm kind of out of ideas (we're new homeowners so still learning about how all of this works!)
Sorry that this post is so long, but any ideas would be greatly appreciated!