Issues with electromagnetic interference on your radio most of the times occur due to electromagnetic fields in the vicinity of your radio antenna or speaker cable. These fields are generated by various sources, mostly data cables with high datarates (=high frequencies)
What generally helps:
- take a look at the shielding of your radio antenna and the shielding of your radio speaker cables. In old cables, the shielding connections tend to oxidize and this makes shielding less effective.
- try to avoid data and antenna cables parallel to your radio antenna cable/speaker cable. Use as short cables as possible.
- make sure the GND-connections (minus) of your devices all connect to one common ground point in the aircraft. Avoid serial GND connections of devices.
If all this does not help, you may resolve the issue by increasing the squelch threshold in your radio. This leads to reduced range, be carefull!