When running water in the bathtub (particularly the cold water), there is a VERY LOUD humming sound which lasts for about 10-15 seconds, and seems to emanate from somewhere between the tub and the basement. This is a very frequent happening, and takes place just a few seconds after the knob is closed. The sound is so loud, and rumbling, that it literally vibrates the floor (and I exaggerate not even in the least).