Motor
All dishwashers are powered by an internal motor that becomes noisy when it breaks down. Problems occur when the seal breaks down and water is allowed access to the motor and bearings. When this happens, it removes the grease needed to properly lubricate the motor and causes a loud noise when the dishwasher is in use. To fix this problem, you need to replace the seal or bearings before the motor experiences more extensive damage.
Inlet Valve
The inlet valve of a dishwasher is what allows the water to enter the tub. If the valve is broken, it can cause a distinctive high-pitched clattering or thumping noise as the dishwasher is running. A general thumping noise doesn’t indicate a problem unless it occurs when the dishwasher is filling up with water at the beginning of the cycle. This would indicate that the problem lies with the water inlet valve. Look for any broken parts of leaks in the valve when examining it. You may need to replace the valve if such damage exists.
Squealing
If you hear a high-pitched squealing noise, it could mean a problem with the fan. When the fan bearings are worn out, it means the entire fan needs to be replaced. You can tell when the fan is the problem, when the squealing noise only happens during the drying cycle. The fan is used to generate heat and distribute it throughout the dishwashing tub.
Minor Possibilities
When determining the cause of the noise, wait through a couple more full cycles of the dishwasher to see if the noise occurs more than once. A loud noise that only happens during a single load could just be the result of a loose utensil or other implement rattling around during the wash cycle. At the end of the cycle, check to see of a hardened piece of food is rattling around the bottom of the dishwasher.