The correct fluid level is probably marked on the probe on the inside
of the reservoir cap.  If the reservoir is translucent, then the
correct level is marked on the side of the reservoir.  If you looked
in the reservoir with the vehicle on level ground, then it sounds like
the level is low (very low, in fact).  This will cause the "humming"
noise, and will pretty quickly destroy the pump.  Get out the owner's
manual and see what it says about changing the fluid.  I'll bet it
says nothing, meaning it doesn't need to be replaced, ever.  Just
topped-off to replace seepage/leakage.  Once you let the fluid level
get low enough that the pump starts making noise, it is unlikely that
the noise will stop until the pump is replaced.