Amy, how old is the toilet and does it have original tank refill parts ? maybe the refill valve rubber components are all worn out and it needs to be rebuilt or replaced ...post back with more data....if it is still flushing and refilling well but is just a little noisy as it refills the tank I would not bother it ...Dont fix it if it isnt broke.
If the refill valve is not shutoff completely but continues to slowly fill the tank to cause it to dump excessive water thru the overflow pipe then that is a definite sign that the rubber washers in the valve are shot and need replacing. Hope this helps. LJ