Why does my toilet make a humming sound after I flush?

Why does my toilet make a humming sound after i flush?

I'm assuming you have a float ball ballcock.(see image).
Noise = vibration. If the adjustments are OK then you have a washer that's vibrating against the seat. Try this. Shut the water off under the tank, remove the three screws securing the float ball and washer assembly and put vthe assembly up. Pry the washer out and reverse it while seating it in firmly. Now place a pan over the open ballcock and turn the wateron for a few seconds to flush out the supply. Reassemble and give it a flush. Hum all better now? Let me know, Tom