Perhaps some details will help:
- You went from no static to static. Anything change, any maintenance or did the noise just start?
- The engine starts as before? If so this confirms a good ground.
- The noise goes away with the alternator off. You replaced the alternator and this same noise is there with the alternator on. I assume your alternator output voltage is correct. If so I’d guess this to be voltage related, and the alternator is not the source.
- Have you tried pulling all breakers other than one radio at a time to see if the static goes away? If not, start with whatever you are using for a USB charging port.
Let us know what you fine out.