Grumpy_Mike:
Can you post a schematic. At a guess I would say it was a lack of decoupling on the op amp power supply.
I have .1 uF decoupling capactitors. it's a quad op-amp chip, that's why I only have decoupling on one of the op-amps but they are, it's all on the same chip. The red box shows what the oscilloscope is reading (the one with the noise) which is what's going to the arduino Analog Input and the green box is the one that if measured does not have that noise and is just a consistent 0.2v, the green box goes on to additional op-amps.
