Noisy Whirlpool Dishwasher

[Thorzdad]

About 2 months ago, our previously silent-as-a-mouse Whirlpool dishwasher started making a very loud, rough, whirring noise throughout the wash cycle. It seems to cycle slightly with the apparent spinning of the wash arms. Water seems to spray fine and the washer empties fine. No standing water. We have begun to notice, though, that the plates are coming out with a very fine, slightly gritty film. Some plate will occasionally have areas with dried-on debris...looking a lot like digested waste. Overall, though, everything comes out perfectly clean. It's just terribly noisy. We also think the wash cycles have gotten longer, but we can't prove that.

I'd like to be able to troubleshoot this myself, but can't figure out how to dis-assemble the spray arm assembly to look inside to the impeller and motor. I also notice that the lower spray arm does not spin smoothly. When I spin it by hand, it wobbles wildly. I can't imagine this is normal.

Any insight would be greatly appreciated. Instruction on dis-assembling the lower arm unit would be especially appreciated. There appears to be a tab on the top of the lower arm that looks like something that should screw off (like the lid of a jar) but I can't get it to budge.

Model# - GU2400XTPB1
Ser # - FR5015685

Thanks in advance!
Jim

Edit: Here's a link to an mp3 of the noise. http://home.comcast.net/~thorzdad/dishwasher.mp3