Using a capacitor to reduce noise on AI signal

[LV_Pro]

If you can look with a scope that will help. What "kind" of noise will help isolate the source, and also provide information on how to filter it. When you say you have shielded leads, make sure that the shield is connected to a good ground, and only at one end, otherwise you may introduce "ground loops" which can themselves impart noise into your signal wires. Also, take care in how they are run, if near large motors, old style computer monitors, parallel to power carrying "mains", that can cause noise as well. Even having a powerful radio station nearby can cause issues. Now, that having been said, 4-20mA is  "less prone to noise", in the transmission phase. So noise tends to be at the head end, where the transducers, etc., are, or after the conversion back to a signal voltage.