Please make sure the GND on Split is correctly connected as one of the following two ways:
If the problem exists still, please try measures below:
1. Change to another VTX. In our tests, some VTXs can produce noise.
2. Change to another PDB. In our tests, some PDBs do not output clean power.
3. Solder one or several low ESR capacitors (making sure it reaches 1500-2000uF）to the board as shown below:
4. Solder an LC-Filter like this to the board as shown below:
5. Power your Split via USB with the TV-out cable and get live feed via this cable：
5. Add an insulating mat between the mount of the Split and the frame
Please check the link for reference: https://www.rcgroups.com/forums/showpost.php?p=38034969&postcount=543.You may also insulate the antenna from your frame/drone/quad.