Alan,
Thank you for your detailed answer. I trust your opinion which is why I wanted to ask you before I sent my Axiom EP500 in. I do understand that sinewaves are torture tests for a driver, but if no one else is hearing it at 20Hz at similar levels, then should this be an issue?

I also understand that the sound may be from the house or objects in the house. I hear that as well, but the sound I'm specifically describing is distinctly different. This sound gets louder as I get get closer to the subwoofer. But I accept the fact that household noises may be mistakenly localized to the subwoofer itself.

As I said, during normal HT listening I have only heard it during one movie, in one scene. This is why I chose to reinvestigate this issue. I will gladly continue to test my EP500 with different HT passages in the mean time. As you said, if I don't hear it during HT passages, then it shouldn't be an issue.

I will probably take to a different room, and possibly outside to listen for the same noise.

Thanks again for your reply.

-Jin

PS I never got your reply or Brent's suggestion...