My toilet flushes fine and the water comes up to the water level in the tank and stops, but 40 seconds later you can hear the sound of water leaking or maybe slowly going back out of the opening. There is no water on the floor, wall, fittings or on the tank. What's going on here>

There is a ball or a flapper that should keep the water in the tank until you flush. It could be bad, or even some crud where it seats. Look it over. I think you can get a new flapper at the hardware store for about $2.

my toilet flushes fine and the water comes up to the water level in the tank and stops, but 40 seconds later you can hear the sound of water leaking or maybe slowly going back out of the opening. There is no water on the floor, wall, fittings or on the tank. What's going on here>
Take your finger and run it around the flat part of the flapper. If it's rough or wavey then it's faulty and needs to be replaced. Also the linkage, (flapper chain) wants to have 1/4" of play when the flapper's seated. Cheers, Tom