Probably just vibration from the motor and pump, being transmitted to the chassis. You can remove the kick panel and see if there are any loose parts that can be tightened. You may want to observe it without the panel, during a wash cycle, to see if it's anything more serious. While you have it removed, it would be a good idea to turn the breaker off and check the wiring connections for loose wires (Dishwashers commonly develop loose connections that can lead to serious problems.).