Depending on board layout, wiring and shielding,
there will always be several possibilities to pick up noise from outside and inside the microcontroller.
Examples are 50 or 60 Hz mains hum may be picked up by wiring loops, or from power supply lines,
and digital noise from the microcontroller clock and digital circuits. There may be other sources around.
If there are op-amps in the analog circuits, then any noise will be amplified at least as much as the wanted signal.
You have a good oscilloscope, so may switch to AC signal display,
and increase voltage gain to study noise voltages in various part of circuit.
A trick that have been used, is to set the PIC into Sleep mode immediately after ADC conversion is started,
and let ADC interrupt signal wake it up when conversion is completed.
This will reduce noise from digital switching inside the microcontroller.
You may program your microcontroller to calculate max value, min value and average value of multiple ADC measurements.