why does the transmission make a high pitch whistling sound

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the van has 188,000 miles. transmission started slipping when i was driving on hwy. i pulled over, checked fluid but it was fine, not burnt in color and no leakage, juast a high pitch whistle sound. started driving again and it started off fine but slipped out of second so i slowed down until it caught again and i just kept driving for another 100 miles at 45 - 55 mph at approx. 4500 rpm