What are you actually looking at? What is the voltage source?
I ask because it looks to me like there could be around 100mV of noise on that trace; the line is blurry, whereas on my Rigol it's pin sharp.
I would repeat this test using two or three 1.5V cells in series (because they are - effectively - electrically "silent"). Keep the wiring well away from noise sources such as nearby LED or fluorescent lighting, switched mode power supplies, etc.