Moan at between 35-40 MPH

So I just got my 'used' 8.75 rear end installed, along with the cut driveshaft. I'm getting a low frequency moan between 35-40 MPH. Doesn't seem to matter if I'm accelerating, or decelerating, it will happen in that range. There is no vibration that I can feel, it remains as smooth as usual as far as I can tell. The rear is used but said to be 'good', but I don't have any information to confirm that. I put new custom axles in it, filled it with fluid that has additive already in it, and ran it. Does this sound like a rear end issue? Any advice?

I should add that the moan is not loud, just like a constant low frequency sound that is loud enough to notice for sure, but not like a bad bearing or something would usually be. It almost sounds like some tires do on certain roads.

Thanks
Alex