CentralCoaster wrote:
Personally I think many of the replacement injectors are louder. Perhaps it's because the injectors are a bit longer and bottom out against the intake base which makes it transmit more noise. A fix for this is putting washers between the fuel rail and base. This is especially a problem with some of the aftermarket intake baseplates. Also I've heard of this binding the injector housings up and causing them to stick open or shut.

Keep in mind everytime the injectors shut, the fuel is still flowing towards them, and it hits this now dead end, and slams into it and makes a pressure spike. This tranlates to noise and vibration at the injectors and fuel lines. If the injectors aren't bottomed out, they should be cushioned a bit from this shockwave (water hammer) by slight movement of the orings.