Touch upâs should be done minimum 4-6 weeks post the initial treatment. This allows time for eyebrows to heal fully before perfecting them for the second time. Free touch-ups are only valid up to 3 months from initial procedure date.
Re: The Tinny sound? Most folks that are concerned about a tinny noise with aluminum boats are referring to the noise generated by the hull moving through the water (which can be modulated/reduced through many different avenues). You mention vibration and rpms, so I'm guessing that you're not referring to that sound.
There are a few reasons why bells are made of metals. First, metal will make bells that will vibrate at a frequency and amplitude that can be heard without an excessive amount of applied energy. They will also sustain this oscillation for a suitable period of time. IOW, metal bells sound like bells.