99Z2ate

So I have a 99 Z28 M6. It has been having some issues via a GM 10 Bolt. I am hearing a grinding or almost a whine starting at 5-10mph. At about 30mph, it turns into a very quite, but high-pitched whistle. It sounds "rotational" at low speeds if anyone understands what I mean by that. It happens no matter if it's in gear or not, that's why I'm thinking that it may not be the ring & pinion.

BTW--It cannot be the diff because I just replaced the Zexel with a TrueTrac and got new bearings & races for it. The sound just persisted.