Hi Jinhan,
There is far more information here than in the email you sent me. Anyway, if you use sine waves or pure tone signals to "test" a subwoofer, and keep raising the level, at some point you are going to cause the driver to go into some mode of mechanical or acoustical distortion or resonance.
Certainly the sound you describe isn't "normal" but then again please use program material (movie soundtracks or wideband music) to see if there is any similar sound from your EP500. If there is, then perhaps there may be a driver defect. Single frequency sine waves put enormous stress on drivers that are not encountered in normal playback.
Here is what I told you in the email, for the benefit of others who may hear "funny noises" that they attribute to the subwoofer: '"In many cases like this, the "odd noise" is often a resonance elsewhere in your room. Our hearing acuity diminishes dramatically at low frequences, and below 20 Hz we don't hear pitch so much as sense pressure. The latter may cause sheetrock, blinds, the frame of a couch, springs in upholstered furniture--almost anything-- to briefly buzz or rattle, and it may seem to come from the EP500. Everything has a resonance frequency and because of the tremendous acoustical output of an EP500 as ultra-low frequencies, it may trigger resonances you would never hear from conventional speakers (which typically have little or no output below about 28 Hz).
It's very difficult to locate these in your room. There is no inherent problem like this with an EP500. Tighten the screws around the driver and, like Brent said, check that no wires are vibrating.
Once you find whatever is resonating or vibrating in your room, you may be able to move it or do something. In most cases, there isn't anything you can do. One of the slats in my Venetian blinds vibrates at very low frequencies. I can adjust the shades and it may go away, but then another object my resonate instead. It can be maddening, but it comes with the territory of serious low-frequency subwoofer operation.
Regards,
