Intermittent mystery toilet flushing sound
Several times a week I hear what sounds like my toilet flushing, however it isn't actually my toilet since there is no water moving at all and the sound isn't coming directly from the toilet but seems to be emanating from below the toilet (into the ground?). This does not happen every day, but several times a week and only lasts for approximately three seconds and will do this noise 4 to 5 times in a row separated by about a half hour to an hour. Then I won't hear it again for a few days. What could be causing this? My home is a single level home and is not attached to any other homes. I've lived here for 18 years and never heard the sound prior to this year and I'm not experiencing any other plumbing issues anywhere in the house.

put some food coloring in the toilet tank to see if the flapper is leaking by. the color will show up in the bowl. this would cause the fill valve to replenish the water and make that type sound.

As suggested, Fluidmaster has FREE dye tablets at your plumbing store to detect a noisy toilet tank. After years of use and agressive waters the bond between the rubber on the flush valve and its base may wear and crack, allowing small amounts of water to pass into the bowl.

If ignore it will continue to be noisy and use water.
Best Wishes