We live in a house 18 months old. Three weeks ago 2 of our American Standard toilets started making a noise in between use that sounds like pressure being released. There is no water leaking into the bowl or onto the floor. Now my water bill has increased by half. There is no pattern to the noise and it is similar to the noise the toilet makes when refilling after a flush but louder and lasts a good 10 seconds. I noticed today that the toilet made the noise when I got out of the shower. Beyond that you cannot recreate the noise.
Are you on public water? What is the street pressure at the meter? (You should be able to call your public water supplier to find out a close guesstimate.)
What is the water pressure at your house? You can get an inexpensive water pressure gauge at most any hardware store or big box that fits on an outside spigot.
If your water pressure at your home is above 60 psi, the toilet problem may be being caused by excess pressure.
If the presssure at the meter is too high (maybe up to 100 psi or more), you may need to install an adjustable Pressure Reducing Valve on your main line after the meter.
If you install a PRV, you also will probably have to install an expansion tank on your water heater. A PRV creates a "closed" system that won't allow hot water to expand back into the cold supply line, and may cause your water heater temperature-and-pressure relief valve to leak.