Toto generally uses two different fill valves. One is easily repaired, one usually needs to be replaced when it fails. I think you should still fall within the warranty period, so I'd call Toto. If the valve you have has a plastic screw on the top, other than cleaning the filter, if it fails, it must be replaced. If it doesn't have that screw, there's a seal that probably will fix it - it takes less than a minute to replace once you've done it the first time.
While you're dealing with this, you may want to check your incoming water pressure. None of the toilet valves like pressure out of range, and in the USA, the max it should ever be is 80psi. If yours is over that, code requires it to be regulated to 80 or less. Excessive pressure can cause that, too. Most common though is either dirt on the seal, or a bad seal. At 8-months, though, it's probably dirt, and removing the seal and cleaning it off will probably fix it, if it is the repairable version. Check the wwhttp://www.korky.com/installinstructions.html site for instructions. Scroll down to the Quiet-fill replacement cap.