Just for funsies check the fluid level in the tranny. Won't hurt to check. Also verify the axles are fully seated.
If the gear box has a helical differential that may account for the noise. I noticed a similar sound coming out the pumpkin of my 240 when I replaced the worn out Kaaz with an S15 helical. Sounded like a whirring, hissing sound. Switching from dino to synthetic gear oil actually made it a bit louder at low speed but quieter at highway speeds.
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