First of all, the audible frequency range is from 50 Hz up to a variable upper limit (~10 kHz, but changes with age and varies individually). Since you usually do not move anything in air at more than 50 cycles per second, it is understandable that you cannot produce sound. In any case, at 50 Hz it would be barely audible. You cannot move an object at kHz frequencies. On the other hand, a guitar string can vibrate at such frequencies, and arguably you could say that it ```taps'' the air at hihg frequency and *does* produce a sound.

So how do you produce sound by hitting a metal? First a metal is hard, so it is capable of vibrating naturally in a large range of frequencies, including frequencies in the kHz range, which are the audible ones. But how do you excite them? To this end, if you remember about harmonic oscillators, you can excite a significant signal in a harmonic oscillator if you use an external stimulus with similar frequencies. Now when you tap against a metal, you ar stopped on a very short time scale. Indeed, the timescale at which a hard body is stopped by another is milliseconds. A signal which lasts a few miliseconds has a components in the kHz frequencies, and is thus capable of exciting vibrations in this range, which are quite easily audible.

A test of this point: if you hit a metal with a soft object (say the fleshy part of your finger) you will get almost zero sound. That is because the soft object is stopped much more slowly than the hard one, so no high frequencies are generated.

Just think of a fan, each blade "taps" every certain time, and there is a sound in the frequency related to the number of blades and the rpms.

Hello Sable, if you close firmly a paper bag filled complete with air and hit suddenly on it, you will get the sound of an explosion. It is not the paper making the sound, but the compression and decompression of the air. So you can concentrate by a bag the energy of your moving hand and tranfer it to acoustic wave energy in hearable range..