If your toilet tops itself up every few minutes, or runs in little bursts through the night, the tank is slowly leaking into the bowl. Nine times out of ten it’s the flapper — the rubber seal at the bottom of the tank. After a few years in treated water it warps and stops sealing, and it’s one of the cheapest parts in the whole house.
To confirm: put a few drops of food coloring in the tank, don’t flush, and check the bowl in 15 minutes. Color in the bowl means the seal is passing. Swapping a flapper needs no tools — unhook the old one, clip on the match, adjust the chain with a little slack.
The other suspect is the fill valve, which can hiss and run if its washer is worn. Either way, you’re usually looking at a small part, not a new toilet. Prefer someone else under the tank lid? CMG Bridge, Staten Island — $125 covers the first hour.