Theoretical: Would it be possible to have a worldwide AC standard at 55hz
Frequency directly affects the speed of some kinds of motors so you can't just change it willy-nilly without affecting a lot of expensive equipment. Low power electronics usually can work with any frequency from 50 to 60Hz (and in fact may work over a much wider range). Making a transformer (mains frequency type) work at either 50 or 60Hz incurs a small (but significant) cost and mass penalty on each unit. 

The hum from pickup of AC line power can be at mains frequency (eg. 60Hz), perhaps plus some harmonics, but the audible hum directly from electrical transformers, old-school fluorescent ballasts, motors and so on is typically double mains frequency (plus some harmonics). 

Don't worry, there will not be a worldwide standard any time soon. The trend is for more and more relatively small devices to accept either frequency. The biggest factor preventing changes in the supplied power is probably the billions of dollars worth of turbines, generators and utility distribution transformers that are optimized for one frequency or the other. Until relatively recently 25Hz was (maybe still is) still used in a few isolated industrial applications.