I Seem To Have A Plumbing Problem?

It is a DIY project.  Close the toilet seat and sit on it in reverse.  Now take of lid to the tank.   What you see is a bunch of rods and pipes...and clean water.  It is the same water you drink...in the kitchen.(so away with "EW" factor)

Now flush and watch what moves when you hold the handle down continuously.  The chain or wire connects to the flapper which covers a 2"-3" opening.

Do not touch the flapper but with your pointer finger rub around the opening on the top.   It may feel slimy. (That is just your water, stuff grows in it. It is not poison.)

Now you will rub around the circle of the opening and in some cases you will see it change to brass.  Sometimes not.  But you are wiping off the slime build up that is holding the flapper open. which allows water to leak past and go into the septic or sewer system.

 After going around it a half dozen times to get the slime off, then let go of the flush handle and the flapper flops back over the opening.  While the tank is filling you can wash the black off with soap and water in the sink.  The black is dissolved rubber. so it washes off easily.  By this time the tank should have refilled. And shut up.   Job is done.  Put lid on and flush. It should be normal. Dead quiet.  You fixed in 5 minutes. Done forever or until it leaks again so several years down the road.       You don't care. you did your job. Never touch the rubber flapper because the rubber has gone out of it.  It is now fragile like glass. Dead flat and fragile.  You finger it and it breaks then you need to buy a flapper the same size(there is 3 sizes).  I don't want to do that when it is perfect NOW.

Handyman 35+ years and had to do this to mine on a 45 year old toilet and on a 60 year old toilet.  Flappers seal up fine.

Bad flapper/flapper seat, Sometimes over time it will get a groove in the flapper seat and leak no matter how many times you replace the flapper.

Sealing surface must be smooth. Gently run your fingernail around the seating surface. Feel for bumps or grooves.

Also, when handling those parts, if your fingers turn black, the rubber is disintegrated.

That is caused by chlorine/chloramine in the water system.

Takes about an hour to remove the tank, replace all the parts, including the rubber washers to mount the tank, and reinstall.

Careful not to overtighten those mounting bolts!

The water goes down the drain. The tank flapper is leaking and it leaks into the bowl, not on the floor. That would be the washer that would let it leak onto the floor.

Put food coloring in the tank water and i bet the bowl will turn the same color within minutes

It could be as simple as the flapper chain is too short because it's knotted up or the flush handle is not allowing the flapper to drop all the way.

Or some object is preventing the flapper from making a good seal.

It's going into the toilet bowl, you've not noticed it because it's a slow, smooth, gentle flow from a leaking flapper.  The level in the toilet bowl is not increasing because any surplus water goes over the bend into the drain.

the most likely problem is the flapper valve at the bottom of the tank.  less often, the seat for the valve becomes damaged.  even less often, the overflow tube inside the tank develops a leak and has to be replaced.  -- grampa

Replace the flapper valve.

The water bypasses the leaky flapper and passes into the pan. You need to obtain a new large rubber washer for the valve. Turn off the water, take off the washer and visit a plumber's merchant. They should have a matching item. Clean up the seating in the cistern and ideally apply a smear of plumber's grease to the face of the washer. This will improve the seal. Grease will be at the merchant's too.

Replace the flapper in the bottom of the tank.

When you operate the flush handle, that lifts up the flapper.  Obviously that water doesn't run out on the floor, it goes into the bowl to clear it.  So the water that is getting past the flapper is going into the bowl.

There is nothing to stop the water in the bowl from running out except the shape of the trapway under the bowl.  It isn't a stopper, just a tall bend.  It's tall enough to hold water in the bowl but as more water goes in, there's enough water in the bowl to push some over the bend, and so the level in the bowl stays the same.

Replace the flapper.