Why does my toilet make a loud,rapid thumping sound when it fills up?

also, concerning johns, mine starting doing something in the middle of the night…when nobody’s in there??????

I would guess the float is sticking a bit, gets submerged somewhat, then slams up when the buoyancy overcomes the stickiness. Does the tank stop filling at the moment you hear the sound?

Your flapper is flapping. what is a flapper? when the tank empties, the flapper falls down and covers the hole that allows water to escape to the bowl. outside water then begins to fill your tank for the next flush. if the flapper is not seating over the hole, you will hear this flapping noise. this generally means you are wasting water, because of a lousy nosie-making flapper.

See above. Either the flapper or the float. The easiest do it yourself fix on earth. Go the local home repair store and tell the guy you want to replace the stuff in the tank that makes the toilet flush. Read the directions. Do it. Cheap and easy.

Flush the toilet. As it’s filling back up, shuddering and thumping, slowly turn off the water supply valve behind the toilet. Not completely off but just until the noise goes away. The noise is probably debris in the line or the water pressure changed in your area. Sometimes turning it off and on while it is filling will clear the line and stop the noise.Even if this fixes the problem its a good idea to install a new flush valve and supply line.It’s always a good idea to replace the flapper also. I’m partial to the Fluidmaster 400A valve and braided stainless steel lines. The instructions are pretty good and it’s a simple fix.
PS. Don’t over tighten the supply line. If it has plastic ends, it can crack.

Mice humping.

Might be air hammer, too. Replacing the supply line could help. The new ones are flexible and very easy to install.

Time to replace the fill valve. It’s cheap, and not too difficult. Get one that has the float integrated to the side, no long arm float. Enjoy your simple plumbing learning experience.