I have five of these in our house, and so are familiar with them. They're actually a very simple valve design with a minimum of working parts, and usually can be fixed with just some easy service you can do at home.
For your specific problem, that noise, first check and make sure that your water supply knob is turned all the way open.
Then, if that doesn't solve it, time to service the valve. What you are going to do is turn off the water, pull the refill hose off the valve with one hand while you steady it with the other (and don't twist the valve; you can twist the hose to remove it). Then, pull the (white or blue or green or silver) cover off the valve, pinch the little arms attaching the float to the valve cap like you're pinching tweezers, and lift it straight up and off. Then, holding the valve body with one hand so you don't inadvertently unlock it, twist the little cap off counterclockwise 1/8 turn, remove it, turn it over and look at the pink diaphragm in it. Are there blisters? Is there gunk in it? If gunk, give it a good rinse off. If blisters or it looks worn or you can't push on it around the little black pin, you can replace it with the Korky R528 cap, available at Lowe's and many local hardware stores for less than $3. Then, under the cap there is a long tube attached to a round top. Pull that straight up and out. On the bottom of the black tube is a white plastic filter. Pull that out, either by hand or with a needle-nose. Rinse everything. You have now basically seen all the working parts of the valve; the only moving parts are that float and the pink diaphragm, which is why it is so easy to service.
Ah, I forgot to add. Even if the diaphragm isn't perfect, rinse everything off and reassemble the valve and see how it goes. That may fix it for now before you need to go buy one of those caps. Then get the R528 next time you are at Lowe's or equivalent, and change that out at your leisure.
I have given you the basics on the truly important stuff, but here are some reference materials that have illustrations, and even a video:
Here's some nice illustrated instructions, where the "Type B" valve is a Korky 528:
toto instructions on how to service a 528
Here's a video by korky on how to service the valve:
Korky 528 Service Video
Also, for what it's worth, you can always call Korky. They will actually walk you through solving any problem and are very, very nice. Their website, which has a phone number, is
www.korky.com. Nice web site, but if you want to call them ever, the customer service phone number is 1-800-lavelle. (Lavelle Industries makes Korky products.)
Let us know how you are making out.