I finally figured out that the buzzing sound echoing through the water pipes in the middle of the night was coming from the back-flow prevention valve when one of the irrigation zones was on. A little research led to the Phoenix.gov listing of companies authorized to to test and repair such valves. I cross-referenced that with Yelp ratings and came up with Forrest Anderson. Not a lot of reviews, but all 5-star and all looked legit. Actual service was beyond expectations: fast, courteous, knowledgeable. E.J. found the problem and REPAIRED the valve, replacing an internal part rather than the whole (expensive) valve. How rare is that these days where most shops can't fix something as simple as a cracked side-view mirror without replacing an entire $600 module. He also provided some help and advice with a situation our cabinet installer encountered around the dishwasher. We are impressed with both E.J. and with the way the company is run. They are now at the top of our list for any future plumbing issues.