ADC noise when sharing ground from external supplies
You've created what's known as a "Ground Loop".  You have 2 grounds in your circuit, your 24v PS has a ground and the PC supplying 5V to your Arduino has a ground.  They are not likely to be equivalent and will cause many unwanted side effects.

You can reduce some noise using RC or RLC circuits on your power supply pins.  It's probably best to use only a single power supply (and ground).