Welcome to Just Answer.
This is an easy fix and will cost nothing, but a little of your time.
The noise you hear is caused by the vibration in the stem and washer assembly.
Shut the water off under the tank and remove the three screws securing the top of the ballcock assembly, that is inside the tank. Pull the float ball, stem and washer assembly up and out. Pry the washer out and turn it over pressing it in in firmly. Place your hand over the open ballcock and reach down and turn the water back on to flush out any crud that might be in there.
Reassemble and give it a test flush. The sound should be gone.
If you should need further help on this question, please reply to this post.
Best of Luck, Brian
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