Humming and slight vibration

Lets see have a 96 E-350 Diesel that has been leaking power steering fluid now for a while. I do keep the power steering reservoir full. Recently I have noticed that sometimes I hear a quiet humming and vibrating that cycles about once a second. When stopped if I turn the wheel the humming sound increases and the vibrating can be felt in the steering wheel and elsewhere. Note this system is not always observed. Suggestions???