With everything in good shape, I have had the squeal/chirp from a perfectly good belt once a drop of coolant touches it, from washing the engine, and from deep water crossings. No amount of cleaning the belt rectifies the noise.
When it's making that noise (sounds like the waterpumps going out) you can verify it's the belt by spraying it with WD40 while its running and the noise will instantly go away for about 30 seconds.
I have found only one cure, Goodyears Gaterback belt. You can pour coolant on it, car wash it, water crossings, no problem. It's got diagonal grooving on the ribs that keeps that awfull squeal/chirp harmonics from happining when the regular belts get contaminated. Costs a little more, but worth every penny.