Don't know weather this will help or not, but I had a very similar issue on a B737 which we eventually traced to a N2 tacho connector, when we changed the connector the problem went away! the connector was a cannon metal type and I can only assume that it was generating some RF, It would be worthwhile checking in the engine compartment to see if you have any similar metal connectors and if you are able to disconnect it and check to se if the problem goes.
On the 737 it took us over 6 months to find the defect and many things had been changed including the aerial, feeders, tr/rx intercom.
Hope this helps
Dave