You could have a little debris in the valve. It is a quick action to take it apart and check the seal, but it may be normal. Many toilets make a gurgle from the drain during the flush, but if it is after that and from the refilling, then it is from the valve. The Korky valves Toto uses are usually pretty good, and don't make much noise at all.
I'd check if any dirt was in the valve, and then, if it didn't resolve the problem, call Toto...they'll probably send you a new one. Toto doesn't make that part, and that valve has a pretty good reputation, so I wouldn't worrry about it.
Just how loud it this? And, how long does it last? When in the refill cycle does it occur?
Note, all toilets are typically overfilled. Think of pouring water into a glass after it is full. That's what happens in a toilet, so once the bowl is full during the refilling, the water coming in trickles down the drain. Some toilets' design allows you to hear it more than others, but it stops once the incoming flow stops after a few seconds. If this is what you are hearing, it is normal, and there is nothing you can do about it.
https://terrylove.com/korky/