Did it do it before? Did it just start? If so, it might be one of the noise filtering capacitors usually installed to supress some of that whine.
In old airplanes it's often a "ground-loop" problem. Nothing to do with taildragger groundlooping. The electrons spend some of their time flwoing though the airframe (ground) and when the airframe gets old and oxidized or loose or whatever, some of the flow finds easier paths. One of those is via headset and microphone jacks, and the intercom or radios pick it up and feed it to the headset. Jacks should be insulated from the airframe using insulators made for that purpose, and the grounding of the jack should by via a separate wire, or the coax shield if coax cable is used.
Those portable intercoms are often found screwed to some part of the panel or floor. That grounds the jacks to the airframe, and I've stopped the whine just be taking the screws out and using Velcro or whatever to mount the thing.
