What Causes Internal Vibrations In Legs?

For over a year now my mother has been having internal vibrations in her legs that she describe as so bad that she can hear it roaring or humming in her ears. She says that it just seems to happen out of the blue and last for quite sometime, as much as a couple of days. What could this be and where should I take her for help?she says that it is not painful but she can t function well when it happens.