Alright;
W58's are known to be noisy boxes that whine like a bitch, doesn't seem to affect their durability much but most seem to whine after 200k etc
If your getting a whining noise while revving in neutral its either coming from the gearbox or its engine related. Have heard electrical interference from the alternator make noises too.
Diffs are normally pretty easy to spot, normally the backlash is shot and they are quiet on overrun. Not always the case but that's how most are.
Can be a heap of things;
Flogged uni's
Flogged center bearing
Flogged diff
Driveshaft out of phase
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