I plugged 2 of my old man's acoustics into my pod today, just for giggles (both have undersaddle piezos).

Both of them had the same low frequency grounding noise when i plugged them in - the only way to stop the noise was to touch the metal part of the jack.

Why is this noise occuring? I'm pretty sure the guitars work fine, as i tried out 2 of them just to be sure, and he plugs them in regularily. This noise was not present when using my electric guitar plugged into the pod with the same cable.