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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.
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Best of Luck, Brian
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This may help:
Please be sure to accept Brians answer. He is correct.
Thank you for asking your question on Just Answer. Brian may have supplied you the answer needed, if so please accept his answer. He may also be correct on why the noise is the problem. I would like to provide this additional information. There are several type of toilets and also different type of fill valves. Many toilets have the type fill valve that Brian described the repair for. Others do not. If your toilet has a fill valve it is often easer to replace them than to repair them. The video below shows how that is done.
Click here if you cannot view the video.
Should you require further clarification, never hesitate to ask. It would be my pleasure to clarify my answer to your question. If after reviewing the above information, you have additional questions, I will be glad to answer them. My goal is your 100% satisfaction with all questions I answer. Please click the green ACCEPT icon if you feel I have given a complete answer, and you are satisfied with my answer. Positive feedback and bonuses are always greatly appreciated!Regards,EdwardAs always when using Just Answer your accept is required for me to be paid.Please do not accept unless you are 100% satisfied with the answer.If you accept my answer please leave feedback.
As I stated above, if Brian's answer is what you were looking for please accept his answer.