There is likely some dirt or sediment in the fill valve. Someone just posted a link to a nice piece by Toto on how to "service", i.e. clean, the valve to eliminate this. (Whenever I am away from my apt for a couple of weeks, I am amazed at how much sediment and junk comes out of the pipe when I open a faucet for the first time; some of that may have gotten caught in the valve.) Here's the official Toto piece: Link To Toto Fill Valve Service Instructions If you have the type B (Korky/Lavelle Made In USA) Valve, here's a video by Korky on how to service it (similar to the Toto instructions, but the video helps): Link to Korky Video On Servicing Their Valve
Usually just cleaning is good enough. If you need to replace the diaphragm cap on the Korky valve (Part R528), you can get it at Lowe's or HD for about $3 (or online or at many ACE's).
Let us know how it turns out. Usually just getting the gunk out and reassembling solves the problem.