angie, the flapper in the bowl is probably old and hard, warped, etc. an easy way to see if the tank is leaking into the bowl is to sprinkle comet or the like around the dry area of the toilet bowl (above the water level) after the toilet hasn’t been used in a while. if the ajax gets wet, remove your flapper,(the thing the flush lever lifts up to get the water going) and take it to the hardware store for a new one. js
Agree with Jack. We used to put food coloring in the tank and see if the water in the bowl changed color. Most newer ballcocks wait until the level in the tank drops to a certain point and then are fully open. This produces the sound you hear. If you take the top off the tank and rub the flapper (in the very center of the bottom of the tank and attached to the toilet handle) you will probably get a black residue on your finger. As Jack said, replace the flapper.