I'm having the same problem. From what i read first check if something is caught in the chopper. If not it may be a bad motor. Check out these links:
http://applianceguru.com/forum1/16048.html#
http://fixitnow.com/wp/2008/09/25/how-to-replace-the-wash-motor-assembly-in-a-whirlpool-built-dishwasher/
http://forum.appliancepartspros.com/dishwasher-repair/7839-whirlpool-dishwasher-makes-loud-howling-noise-during-wash-cycle.html
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November 2008