Sounds like water is leaking as it fills. It's probably around the tube as it refills or it has come lose and is spraying the inside of the tank. Take off the tank lid and watch as it fills. Easy fix. The inside of a toilet is very easy and simple to fix or replace. Worst case is a new assembly. The whole insides can be replaced for a minimal amount. My husband gets a kick out of the fact that I replace the guts on our toilets as they go. Turn off the water to the supply underneath and go for it.