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The most likely cause is that your toilet’s flush valve is leaking through the flapper, letting water continue to drain from the tank down into the bowl. You can check this by pouring some food coloring or blue fabric softener into the tank. Let it sit for 10 minutes than check back to see if the color of the water is the bowl has changed. If so, start by making sure the flapper is seating properly, and replace it if necessary.