Glad to hear that got resolved so easily. I suspect that the pipes themselves were fluttering and that's what you heard. Can't say for sure from here, but if there's a swing/flapper type valve at the WH......it can cause that under certain pressure/flow conditions. The valve itself will then/can...... rapidly flutter open,shut,open,shut....which will in turn cause the pipes to vibrate.
Or.....the vibration may have been generated right at the stop-valve in your case.
Can't help but think that the copper dealie you saw at the store was a hammer-arrestor. Spring-loaded likely.
You say you want this to turn political? Be patient cause I'll have to work on that. Doesn't come natural to me.