Plumbing Questions? Ask a Plumbing Expert.
The noise is from the fill valve. The seals are becoming worn and the water is "whistling" as it passes through the valve. Simply replace the valve, it will solve the issue. you can get a replacement at any hardware store for $10 or less. I recommend the fluidmaster brand. Instructions on replacement are on the boxes and is very easy to do, but if you have any questions just return here!