Howl from rear end

At first I thought it was the hub and had that replaced last year but it’s gotten worse so that it was a humming noise and now it’s a howl. Starts at higher speeds so I’m thinking it’s the rear axle. Maybe the struts? Big brand tires took the wheels off and said there’s nothing that would be a safety issue then their tech got in it and drove it down the street and said that yes he heard it too but didn’t know what it was. I thought maybe the guy who worked on it last added too much tranny fluid to it and it showed way up on the stick way past the full mark so not sure if it might be that? It’s coming from the middle of the rear end. I also thought maybe it’s just the bushing?