You may have a blown midrange. It happens with almost any driver if you overdrive them or feed them to much clipped/distorted signal.
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I am not sure this would help. I had similar problem in my energy speaker. After trouble shooting a little, I jumped to the conclusion that the speaker has the problem. I actually removed the speaker for fixing. In the end, it was one of the tubes in my amplifier. If you have a tube amp, you may want to make absolutely sure it's not any part of the amp first.
Thank you shadorne and s100. I have luckily resolved the problem. There is actually some dust (or small insects) stuck in the tiny opening around the center part (not sure how it gets there another theory is the glue comes off a little bit). So when the unit move back and forth, it makes noise. I managed to remove that using vacuum and the problem is gone.