squealing belt, '87 300sdl

[4givnsinnr]

New Continental belt on engine now, less than 200miles since replacement. After removing old belt found idler pulley bearing on tensioner ok, water pump and alternator turn freely; the squeal, is the belt; using a squirt bottle, engine running, I can spray a little water on the underside of the belt, squeal goes away immediately but comes back soon. I don't want to use retail "belt dressing" to treat the symptom, would prefer to treat the source. Anybody know the source???