Don't remember if this one uses the Unifit adapter. If it does, you'll get that, but usually not that long. A typical toilet is all procelain, and it damps the sounds better.
When you flush a toilet, filling the bowl is like filling up a basin with a pouring spout...you can only fill it so much, and it overflows. A toilet bowl is almost always filled to overflowing, with the excess (hopefully not much) going out the overflow and down the drain. If the wind is blowing, it can change the air pressure in the drains, and rock the water...causing a little bit to splash out the overflow (called a weir) in the bowl. Eventually, enough leaves so that can't happen anymore. On a normal toilet, you probably wouldn't hear it, and you won't on this one after you realize it isn't broken. It happens when you are putting stuff into it as well, remember, it's a full bowl, add anything, and it begins to overflow.
It could be somethign else, and is if it doesn't have a Unifit adapter. Supposedly, Toto is going to build future (or current new) ones with some noise damping material on the plastic unifit to lessen this effect. I no longer hear it unless I listen for it on mine...sort of like not hearing the grandfather clock tick after a short while.