Its a wifi probably. Bluetooth maybe also...Rpi is close in distance to other electronics. The transmition intensity is too high for such small proximity and is picked up.I would guess RPI works on 2.4Ghz wifi. Make to work it on 5Ghz and the problem can be gone(Easiest way to make it in router settings). If that doesn’t help external usb wifi dongle could be used to move away the transmiting antenna phisically from sensitive electronics.
I know I am late to the discussion. The RPi's are particularly noisy. I read a story about a hunt for a transmission at the national quiet zone in Green Bank, W.Virginia. They have a team which searches for any cell phone or other transmission. Turns out it was a Rpi, quite a ways off.
What you can do is to feed both sides of the circuit through a common mode toroid. Also add caps to the power side Put it in a metal box and ground it to the rpi and ground it in the car.
The emissions can be through the air, through the wiring or through the Chassis of the vehicle.
If you are powering the rpi with a DC-DC switching power supply, get rid of it and use a linear regulator, ala lm317 or 7805 type. Have the power come directly from the battery to your regulator.
What you can do is to feed both sides of the circuit through a common mode toroid. Also add caps to the power side Put it in a metal box and ground it to the rpi and ground it in the car.
The emissions can be through the air, through the wiring or through the Chassis of the vehicle.
If you are powering the rpi with a DC-DC switching power supply, get rid of it and use a linear regulator, ala lm317 or 7805 type. Have the power come directly from the battery to your regulator.
Even later in this discussion... I have a weird RPI interference on FM that manifests itself as a whine that increases in pitch as I tune up in frequency. If I pull the ethernet cable from its socket it disappears. I believe this is an idle signal the ethernet interface puts onto the network cable. I tried using a ferrite clamp on the cable but as it is a differential signal this does not help. I believe the WiFi/BT interface is innocent as this is a different frequency range. I use a model 2B.