Flight computer - radio interference

One of my friends is bothered by his Becker radio, emitting terrible
noises through the speaker, whose origin seems to be the flight
computer (LX7000).
The power supply to both devices is fitted with ferrites.

I tried a few tricks, like wiring the metal case of one or both
devices directly to the negative or to other metal parts of the
cockpit. No effect.

One thing is a bit disconcerting for me: as I removed the antenna
cable from the radio, the noise stopped immediately. The antenna is
integrated in the fin, pretty far away from the panel. The antenna
cable looks like new, it is not bundled together with other wires and
isn't too long.

Anybody have any suggestion?

thank you very much!

Aldo Cernezzi