Motor

• Dishwashers are driven by an internal motor that gets quite noisy when defective. The problem occurs when a seal breaks down and allows water to access the bearings in the motor, removing the grease needed for proper lubrication and creating a loud motor noise when the dishwasher runs. The seal or bearings need to be replaced in this situation quickly or more extensive damage to the motor may occur.
  
  

Inlet Valve

• If the inlet valve on your dishwasher is broken or breaking, you will hear a distinctive thumping or clattering noise whenever a load is run. A thumping noise doesn't always indicate this problem, however, depending on when you hear it. If you hear a noise like this when the water is filling up the dishwasher at the start of the cycle, the noise likely points to an inlet valve problem.
  
  

Squealing

• A high-pitched squealing noise is an indication that something is wrong with the fan. Fan bearings that are worn out indicate that the fan apparatus needs to be replaced. If the noise you're hearing is in fact coming from the fan, it will only occur when the dishwasher is drying the dishes. Dishwashers use a fan to generate and distribute heat to dry the dishes quickly.
  
  

Minor Concerns

• Sometimes even a minor concern can cause a great deal of noise. The thing to watch for is whether or not the noise repeats itself on multiple loads. A loud noise on a single load that doesn't repeat could be caused by things like a loose utensil or small implement rattling around loose in the dishwasher. Also inspect the bottom of your dishwasher. Hardened bits of old food can become rock hard under the heat in the dishwasher and cause noise when the unit runs.