toilet thunk

I replaced the flapper in a Kohler elongated bowl toilet that is 10 years old because the old one was hard and every time you'd flush, it went thunk. I replaced the flapper with a universal type. It still goes thunk. To prevent and cure the thunk, do I need to use a flapper like the original (with a float on it)? If not, what is my recourse?

Is it a thunk when the flapper drops, or does it thunk when the tank finishes it's refill.

Sometimes the Fluidmaster fill valve creates water hammer.
Replacing the Fluidmaster with a Korky Quietfill will fix that.

the "thunk" in the night

the thunk happens once the tank is full and it makes sense that it is a water hammer. thank you for your VERY helpful answer, and quick reply. a korky quietfill is on my list for the supply house.

Ah ok this helps..
The same thing happens in my 1997 American Standard Cadet 2.. I replaced the old American Standard Fillvalve with a FluidMaster Whisper Filllvalve with LeakSentry..and I get the Loud thunk when the flapper goes down.