The 38mV noise that you are seeing on the +/- 5V scale is the inevitable noise built into all 8 bit resolution scopes. At 10V pk-pk voltage range you can figure the lowest step the ADC can display by dividing 10 by 256 (the number of steps in 8 bits). If you step up to a 12 bit scope this would be decreased down to 2.4mV (10/4096).
Since you are measuring a DC signal and simply wish to measure the noise, turn on AC coupling and then you can lower your input range down to the mV scale...
Crag Technologies, Inchttp://www.pc-oscilloscopes.com