I will take this opportunity to tell my tale of plumbing drips-just because is was a drip and so unusual. It only happened in the spring and fall and resulted in wet spot in the carpet in the bedroom adjoining the bathroom.
Turned out the condensation from the vent pipe was the culprit. The vapor only condensed in the vent pipe in the cool spring and fall and only at the very top of the vent pipe where it went through the roof. There was a bad joint in the vent pipe lower down and the condensation would run down the inside of the pipe until it got to the flawed joint where it could seep out onto the outside of the vent pipe. From there it ran down the outside of the pipe to the slab where it happened to seep out into the bedroom. Took me a season or two to find that problem.
If by “flushes itself”, you mean the fill cycle initiates without your flushing, the most likely cause is a leaky flapper valve. If you have replaced the flapper, it could be a pull chain that is just a tad too short (preventing the flapper from fully sealing), the flapper is not installed correctly, or the flapper seat is damaged.
First, examine the chain. Make sure there is slack when the handle is at rest. If that is OK, then turn off the water to the fill valve and remove the flapper. Feel the seat, where the flapper seals at the bottom of the tank. You are looking for a nick or break in the material. If that feels OK, then reinstall the flapper, making sure it is sitting square to the bottom, and see if it still works.
If, by “flushes itself” you mean something different, explain in more detail.
Does the sound only happen when you turn on or off the water? Sometimes the pipes may flex and make pops that sound like drips. Usually they’ll stop quickly after you turn off the water.
An example might be after you turn off the kitchen sink, you hear ‘drips’ in the wall opposite the sink. In that case it’s likely the way the pipes are routed and attached through the walls that cause a bit of flex when the water flows.
