Wall outlet is making a hissing noise
It's rather unlikely the cell phone charger is a transformer these days.  Nowadays, they are switching power supplies.  

While it's possible to build a switcher with .99 power factor, the cheapies have really weird power factor with crazy harmonics, and at much higher currents than you'd expect.  This creates EMF (literally, electro-motive force) which can make things move, i.e. vibrate. 

I could see that being a "canary in the coal mine", causing weird effects on an outlet that's already marginal.  A tight good outlet should not do that.  If that's a NEMA 1 type receptacle, it's gotta be an oldie.  It may simply be a wire that has loosened over the years, or an internal component that has broken.