On my T4 the slipper clutch makes more of a whine noise. It's much higher pitched, and a much "smoother" noise then when the diff slips. I would guess based on your description of the sound (plus, since you already tried tightening your slipper clutch) that your differential is slipping. When the ball diff slips it makes quite a racket.

I had to tighten my diff when I went brushless (and I only went with an 8.5 turn Novak). Also, something to note about the slipper clutch. If you hit reverse really hard on a good traction surface (for instance, hit full reverse throttle from a stop), the slipper has a tendancy to loosen itself a bit. This will only happen if the slipper is loose enough to slip when you hit reverse, but it's just something to keep an eye on when you run a lot of power if you use reverse much.