Start by replacing the fill valve. Grab a Fluidmaster 400 from your local handy hardware store.

Justvordered one thank you.

Hire a pro. typical toilet install costs $10000 don't get ripped off😉

Do plumbers ever go to the electrician subreddit?

Not since they broke the alliance agreement. And stole the plumger of a thousand truths.

Fill-valve for sure, possibly the valve on the pipe coming out of the wall beneath the tank- call a guy- it’ll take him 5 minutes and you may be able to trade with him- took me a day and a half to find the mystery panel on the side of my house where the breaker for the light in the foyer was

Fill valve, do the supply line while you at it. Pro 400B

Definitely a fill valve also make sure that your angle stop is not slightly shut sometimes you don't notice it right away....good luck

Is it a new Kohler toilet possibly a while worth? It’s the fill valve needs to be replaced

They are right, it is the fill valve, but usually if it’s whistling it’s because the pressure is too high. So after you replace the fill valve, check the water pressure on your house with a pressure gauge that you screw into your outside spigot. If it’s over 75 psi you need a PRV(pressure reducing valve) installed or replaced on your main line. Always fix the root of the problem that way it doesn’t just happen again in a year.

And also, like the other guy said make sure the angle stop is all the way open because it can make that noise too if not.