I am not a plumber but if the toilet is flushing a large amount of water has to drop down from the tank. The flapper (or equivalent) has to open wide before a large amount of water can leave the tank. The flapper itself is defective or some other component in the tank is causing the flapper to open at the wrong time.
Are you certain that the toilet flushes or do you just hear the tank refilling partially? If water slowly leaks out of the tank (look closely in the bowl you may see a faint trail of water bleeding down continuously) it will lower the level in the tank until the valve reaches a point that it will let more water come into the tank, and you may be hearing the tank being refilled.
There are many types of flush and refill mechanisms but if water is leaking it probably involves the flapper (or if it is flushing) or other device that closes off the big opening that allows water to pass from the tank to the bowl. A very, very small amount of leakage will cause faulty operation. One way to tell is to put food coloring in the tank and see if it appears in the bowl without any flushing. If it does there is leakage.
Are you certain that the toilet flushes or do you just hear the tank refilling partially? If water slowly leaks out of the tank (look closely in the bowl you may see a faint trail of water bleeding down continuously) it will lower the level in the tank until the valve reaches a point that it will let more water come into the tank, and you may be hearing the tank being refilled.
There are many types of flush and refill mechanisms but if water is leaking it probably involves the flapper (or if it is flushing) or other device that closes off the big opening that allows water to pass from the tank to the bowl. A very, very small amount of leakage will cause faulty operation. One way to tell is to put food coloring in the tank and see if it appears in the bowl without any flushing. If it does there is leakage.
