Lots of good comments in regards to the possible sources of noise, but you also want to look at the victim side of the system. I am not sure what radio brand you have, but most of the principles are the same.
Noise can enter a radio/audio system in one of four ways typically: Through the power wires, through the antenna, through the radio case itself (rare), and through the audio wires from the headphone or mic jacks. And if you are using an audio panel, the wires from it can be added to the list as the audio jacks should pass through the audio panel.
I would look at these in turn and would start with the audio jacks as they often are installed wrong. The jacks for the headset and mics should be insulated from the airframe using fiber washers. This can be checked if you can unhook the connector at the radio or audio panel and ensure nothing on that end is connected to a ground. Then use an ohmmeter to check each contact inside the audio jacks to ensure no continuity to ground. This is one of the most common sources of audio noise.
I would next check the power and ground lines to the radio. These ideally are a twisted pair with the ground side connected to a single point inside the cable where all other avionics grounds are connected. The “forest of tabs” style ground blocks sold by B and C are excellent for this. It is also ideal if the power source is located near the ground point, but that is often difficult in airplanes that use breakers for power. I am using an automotive style PDM in my S-21 so that the “loop area” of my power/ground circuits is minimized.
I would next check the antenna to ensure it is well grounded to the airframe at the attach point. If the paint wasn’t removed during installation, the ground may be poor at the antenna mount. Look to the antenna manufacturer for mounting details.
If you have an audio panel, then you need to check all connections between it and the radio and ensure the manufacturer’s installation requirements were followed precisely. Specifically, the use of shielded cables and proper shield termination. Shields may be terminated at only one end (typically the case with the headphone and mic jacks and many sensors) or may be terminated at both ends which is common in cables that go from one metal enclosure to another (audio panel to radio for example) in order to make essentially one large Faraday cage.
The least likely issue is the radio enclosure itself unless someone has modified it by drilling holes in it or such, but that is very unlikely and the radio makers generally test to ensure that the factory holes in the enclosure are small enough to block any signals that would be harmful.
Good luck and let us know your progress. Chasing down electrical noise can be a frustrating experience, but approaching it systematically is generally the best way to maintain your sanity.
