It took an hour of running back and forth and stopping to read the instructions link again on the computer, but I finally got the problem solved without a plumber. That rarely happens, so is cause to celebrate! It was a combination of too-high pressure from the city (so my gate valve at the curb is now about 1/3 turned to lower the pressure) and an "air hammer." Years ago I lived in an apartment with radiators so had an idea that air in the lines was part of the problem.