Hearing a rhythmic knocking sound while your dishwasher is running usually doesn't mean there is something wrong with the dishwasher. It is most likely caused by the way the dishwasher was loaded, causing the spray arm to hit something inside the dishwasher as the spray arm rotates.
The solution to this issue is to rearrange your dishes, glasses, and other items in the racks so that you're providing clearance for the spray arms. The spray arms must be free to rotate for good wash performance.
Proper Loading Tips
You can find basic loading tips for loading dishes in each rack of the dishwasher by visiting Dishwasher - Loading Tips.
For a quick overview of the best way to load a dishwasher, watch the following video.
Dishwasher Loading Tips
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Additional Troubleshooting
There are many sounds you may hear when your dishwasher is in use, and some may be normal while others may be abnormal. For help with other types of noises you may hear in your dishwasher, visit Dishwasher - Normal / Abnormal Noises.
The video below provide guidance for other normal sounds you might hear.
Dishwasher Normal Operating Sounds
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You should also consider checking your dishwasher's Owner's Manual for more specific loading suggestions and diagrams.