Apogee Centaur Ribbon Mid/Tweeter Replacement

How do I know when I am to replace the mid/tweeter ribbon in these Apogees? What sort of noise/distortion happens that would tell me that they are worn? I am currently hearing almost what sounds like cartridge mistracking in my left speaker. I know that this is not the case. There are just certain frequencies where there is a buzzing noise,kind of like vibrating tin foil (if that can be imagined). There is also an occasional sound like a tube going bad. It ONLY happens when music is going through the speaker. If I just listen with no music, neither noise is there. If you can be of help, please let me know. Thank you!