Drake makes noise after flush/fill is done

It's like when you turn a car off and the engine keeps running for a bit. It's an airy Ch-Ch-Ch-Ch sound, and lasts for about 15-20 seconds after the tank is refilled. It's coming from the green part inside the tank. In other parts of the house, though, it sounds much louder, like a clunking valve sound. Any ideas? TIA.

More Info

I looked in the tank last night and noticed this is an Eco-Drake, not a Drake. Don't know if that makes any difference. Also, the bathroom floor is not even. Not a few wobbles, but the whole floor tilts off to one side.

If the flapper is leaking, then the tank will continue to refill over and over again. Check to make sure the flapper isn't leaking.

If the flapper is good, and the water in the tank is going over the top of the overflow tube, then the fill valve needs cleaning or repair.

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