Even department stores have things that emit a high pitched noise in their stores to deter teens and loitering. Infrasound is also a thing and it occurs naturally or as a result of many things working in just the right way. That sounds drives people insane and they cannot pick up on it.

To me, everything you describe points to some sound actually being there. Not all microphones can pick up all ranged of sounds as well. There may be a sound that it's not able to pick up.

I have really sensitive hearing and my so does not. Even some chargers emit a high pitched sound that drives me nuts while he can't hear a thing. Anything plugged in down there? Could be something behind the wall.

Keep your wife at the parents for a while until she calms down. If she's not hearing anything there after like a month, I'd say definitely not crazy and there's something there. If you take down the wall, you can bring her in to pinpoint where it's coming from for curiosity's sake, or MOVE. It's not worth losing her over