Could my pulse watch be similar to what you're making? I press my finger on a metal plate, and my pulse rate appears onscreen. I have wondered, how can it detect my pulse? All I can figure, is that a tiny voltage differential is generated between my left hand and my right hand. And the watch contains a high-gain amplifier.
I touch the leads of my voltmeter. A few uV often appears. On the ohmmeter setting readings are between 1M and 2M ohms.
A high impedance loop can easily pick up ambient mains hum. You might need to filter it out of your signal.