I get this as well, but it's only when in gear and coasting(5sp). I figure it's just the coast side of side of the ring and pinion and the tranny gears working in the reverse direction. It sounds similar when in reverse.
Sounds like a pinion bearing or loose yoke. If the yoke is loose, you may be able to get away with replacing the crush sleeve and pinion seal, then re torquing the pinion nut.
If you need to replace the pinion bearings, then as stated above, probably wouldn't hurt to do the carrier bearings as well.
Don't "just replace" them. You need to get a picture and backlash reading prior to dis-assembly of the old gears.
Double check the picture and backlash after the new bearings are installed.