Water hammer is wild, right? I used to think it was just air too, but apparently it’s more about momentum—like, when you shut off a valve too fast, all that water still wants to go somewhere, so it slams into the shut valve and makes that knocking sound. Sometimes air in the lines can make noise, but water hammer’s usually just pressure and speed. Not exactly sinister, but it sure sounds like it at 2am.
About those booster pumps, I’ve seen some setups where only the cold side gets a boost, especially if the pump’s installed before the water heater. If you want both hot and cold to improve, you really need the pump on the main supply line before it splits. Did you notice if your hot water pressure changed much? I’m curious if anyone’s had luck with a pump on just the hot side, or if that causes weird balance issues with mixing valves...