It's most likely the fill valve (it could be the shutoff valve, too). The fill valve may just have some dirt/sand inthe gasket. Dependingon the brand of filler valve, it may be a $.50 part and a 2-minute repair. But, you can replace the whole thing for about $5.00 and it is easy to do. You may want to replace the hose from the filler valve to the shutoff at the same time, another couple of bucks. Kory or Fluidmaster are two good brands available most places. My unprofessional opinion.