Water Hammer won’t go away

Our house has water hammer that seems to never go away. After emptying the pipes a bunch of times I called a Plumber whose been by a bunch of times. He can’t find a root cause and installed a bunch of hammer arrestors which have helped a lot but the problem persists (the hammers are less frequent and quieter but definitely still there). He says he’s never seen anything like this, since the hammer persists even when no water is running in the house, and he couldn’t see pressure jump or drop on a gauge he installed. He basically said he is out of ideas other than to just install more arrestors or a larger expansion tank for the hot water tank. Any ideas of things he/I can look for or try? The water hammer really drives me nuts.

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