Just to answer my own question, the vibration is from the vacuum breaker after the Rachio Hose Timer. Inserting another vacuum breaker between the Y manifold and the hose on the other leg results in buzzing from that breaker whenever flow starts from the hose or ends, but not while running. Why the breaker after the timer buzzes for the full duration (rather just when starting or stopping) of the Hose Timer’s watering cycle, I don’t know. It’s a significant source of vibration, which carries into the house.
I’ve tried similar breakers from 3 different manufacturers (Zurn, Eastman, Everbilt)… they all buzz. Most online queries say that’s what they’re supposed to do. The breaker all spec’d for 125 psi limit - not close to local pressure.
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BOOM! Fixed all the drips.
Maybe raising the pressure or increasing the size of the output line might be the fix. The vacuum breaker is trying to open and close at the same time because there is not enough of something.
That something is probably flow. I’ve always had a working anti-siphon directly attached to an Arrowhead hose bib, but it specifically is NOT designed for constant under-pressure use, like preceding a Rachio Hose Timer. There’s even a label on the Arrowhead anti-siphon to this effect. I’ve experienced failure of the anti-siphon device (eventual leaking because of the deformation of the “check valve” material) with an on-for-days pressurized garden hose with a hand trigger shutoff.
The proper place for the check valve is AFTER the Hose Timer, but I’m driving a porous soaker hose, which of course works with high back pressure. Hence, I believe, triggering (alternating on/off, rapidly, loudly) the anti-siphon valve, causing horrendous noise. SpigotMaster makes adapters, some of which I own. For now, I’m not using those or any anti-siphon device.
