MustangGt40th

my right door speaker has been making a rattling noise when i have my bass all the way up when i turn it up pretty loud ever since i bought the car. it thought it might just be loose since it just rattles on certain base frequencies, so i took the door panel off and took the speaker out and retightened all the screws with no success. so im guessing its either busted or on the verge of busted. any tips or ideas? and if i wanted to replace just that speaker because im really happy with the stock system where would i go and how much would it cost?