Static on one freq.
This kind of thing often originates from some piece of equipment that incorporates a poorly designed switching regulator in its power supply. A poor design or one with inadequate filtering can radiate at a whole range of frequencies and interfere with your radios. Many items, including EFIS units and radios now have switching regulators in their power supplies. A give-away is if the unit is rated to run from 14 to 28V without any mods or settings.
With the radio on, try selectively turning off other items and see if the static disappears with one of them. I had an early GRT MiniX that splattered RF noise at 1mHz intervals throughout the aviation comm band, but they have fixed the problem a few years ago. Another item can be a device charger, such as the kind you plug into a cigarette lighter. These often are RF noisy. It can also be caused by many kinds of LED lighting.
I have an LED hand work light that generates so much RF noise that I can't use it in the plane!