Why did static electricity make a humming sound? My son was generating a lot of electricity the other day by rubbing his socks against the trampoline. He could shock others (and himself) with the static. What surprised me was that when he was charged and held a dry twig close to the trampoline's metallic rim a humming sound was emitted. How is this humming produced?
 A: What you heared could well have been the result of corona discharge.  Corona discharge is the result of the air surrounding a charged object becoming ionised because the local electric field is very high but not high enough to cause total breaks down of the air.  It is often of accompanied by a buzzing and a violet glow which can be seen if it dark.
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The static electricity produced by your son could well have produced an electric field around the twig when it was close to the earthed conductor (trampoline time) to cause the air around the twig to ionisé particularly as a pointed object will enhance the electric field strength.  
If is possible to repeat the effect in the dark you might be able see the air glowing as the result of the corona discharge?
A: Static electricity itself won't generate a humming sound.
A humming sound is the result of something oscillating. For example, in the US, you will often hear "buzzing" or "humming" sounds at 120 Hz near transformers or other electronics because of the oscillations of the currents in the power grid.
Static charge, on the other hand, will cause charges to move around once and stay in their new location. For example, if your son had a negative charge and he came near the metal frame, the parts of the metal frame near him would have some positive charge. But this wouldn't cause any oscillations or humming. In fact, this is the meaning of "static" - not moving.
There would need to be other forces at play to generate a humming sound. It's hard to say what those would be, though.
