The squeal is a loose belt, but if it happens again very quickly after re-tensioning, I'd start looking for the cause of excessive belt wear. This can be bad pulleys, a missing crank spacer (this will cause serious oil leakage, though -- don't laugh, we left it out of my brother's car since it didn't have one when we took it apart...), or a bad water pump. If the water pump shaft is moving out of the bearing, the belt will be get pushed sideways -- this wears it pretty fast.

I've also had an alternator bind up internally, causing the belt to wear excessively fast, but you've had the alternator replaced already.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!