I think that it's wear in the final drive.  Our '88 has been doing it too for about 6 months.  Doesn't seem to get any worse (though I'm sure it will).  I drove a Volvo 245 Turbo with a whiney rear end for almost 3 years and 90,000 miles.  I don't think that the end is approaching yet.

Gary
  ----- Original Message -----
  From: Jason Willenbrock
  To: vanagon@GERRY.VANAGON.COM
  Sent: Tuesday, March 20, 2001 4:05 PM
  Subject: whine?

  hey all,
  today i noticed a whine in all gears when rpm reaches about 3800.  is this the beginning of the end for my manual tranny?  how longer will it hang in there?  any ideas?
  jason

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