I have the ground for an SD card module, LED, and piezo sensor all plugged into the same ground rail. When doing this I get a low "hum" in the piezo readings where instead of reading ground voltage (0) when there's no vibration, it cycles between 4 and 6.
However, when I switch the piezo's ground onto ground rail connected to different ground pin on my Arduino, the hum goes away. If I keep the piezo on original ground and move the SD card module's ground, the hum comes back.
I'm very new to electronics, so I'm wonder what's going on?
EDIT: Here's a diagram of the circuit.