The programmer of Linrad, Leif (sm5bsz) has recently been experimenting with some modifications to the Airspy metal case. He discovered that the USB and RF input connections on the Airspy were not making good electrical connections to the metal case because of the paint on the case. These bad connections caused interfering broadcast FM to be received by the Airspy through the USB cable even when the antenna input was terminated with a dummy load. By sanding down the paint on the metal box to improve the connection he was able to significantly reduce the interference. He writes:
It is a good idea to make sure that both the SMA connector and the USB screen have a good electrical contact with the box. Grounding only the USB screen causes a severe degradation of the NF.
The findings here may also be useful for improving shielded RTL-SDR dongles.