Tylerx7fb

I've searched a lot and I cannot find an answer to my question. So hopefully someone has experience with this. Car is a 1984 rx7 gs. The sound is hard to explain, a metallic flat tire noise is the best way to describe it I suppose. At low speeds up to around 20 mph I cant hear it. As I increase speeds it gets faster and louder. My exhaust is rather loud so I can't really pinpoint the sound. I have three ideas: a bad wheel bearing, bad tire, or what I am leaning towards, something in the differential. A subaru tech at my job said the bearings didn't have play, but they wouldn't anyways if they are pressed on right? As for the diff, it is a limited slip and the fluid is pretty decent but does have an axle leak so it got about an inch low so I topped it off. Planned on tearing the rear off over the holiday break but it turns out I don't have much time. I'm going to try changing the fluid completely and see what kind of damage is done if I can get the drain plug out. If I can get the rear off and the diff apart I'll update but input is greatly appreciated