Experienced Plumbers? Lots of details…?

Yesterday afternoon a hammering issue developed suddenly with the water pipes in my Dad’s pre-fabricated, 6 year old home.
I started a wash cycle. It filled up normally. At the point when it decided to re-fill itself for a rinse cycle, there was a hammering sound for the duration of the fill up. When we opened the lid, stopping the water, the vibration stopped. When we closed the lid it started right back up. When we turned on the basin faucet it caused it. When we used the restricted-flow sinks it did not.
The toilets didn’t cause it. And the tub spout must not be tied to the same run of pipe, because it didn’t cause it either.
So it seems isolated to the washroom. Well today it changed it’s occurrence. Now in addition to the above, it sometimes happens for a second when we turn on the kitchen sink quickly to full. And it happened briefly when the shower was started.
We isolated the vibration to somewhere below the fridge ice-maker line yesterday, and that hasn’t changed. I noticed the plastic lines coming off of the stem valve shaking with the vibration.
I looked briefly under the house. It is set on a block wall foundation with steel I-beams and jack stands across the full length of the house. The floor seems to be wood framing (2×12), insulated and sealed with black plastic. Not certain that the wood is solid rather than composite joists.
(I know I’m giving a lot of information that probably isn’t necessary, but I’m curious to know what an experienced plumber would expect the cause to be and want to make sure you can know what’s going on)
My first thought is that a strap came loose suddenly… and then the hammering might be breaking other straps loose. (Perhaps they are degraded plastic clips)
I’m just not sure why the vibration would occur during the running of the water. Isn’t hammering normally related to closing a valve rather than allowing the water to flow freely? It stops immediately upon cutting the water off. The only other thing out-of-the-ordinary associated was that yesterday afternoon (before the laundry) I strung a 100 foot hose from an outside spicket to wash my car.
The hose is 1/2 inch I/D and has a small hole in the line where a mist sprays out.
We’re on county water in a canyon of southern California.
The pressure of the line seems to have risen a bit since the problem started. Currently it’s around 84 PSI. Does this seem normal? My dad says he has it higher "on purpose" (not sure why).
I thought house pressure was supposed to be around 40PSI
The piping is 1/2" sweated copper.
I’m really not looking forward to cutting the plastic sealing wrap or pulling down insulation, and working in a spider-infested 2 foot high crawl space just to see what’s going on in the first place… so any advice (especially finding out preemptively that I don’t have to) would be GREATLY appreciated.
Thanks for reading.

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