Thanks @Gman. I will look into getting a sound meter. The low-tone is not part of my tinnitus. It's an external sound. I just can't tell if it comes from inside or outside of the building. (DH can't tell either and his hearing is normal.) The reason I get panicky is that all this noise is in my home, so there's no escape. Home should be the one place I can be safe and leave my ears unprotected.