Could be quite a few things, but my first guess is a problematic tire. Tires are made in molds from laminates with the patterns of each layer configured in precise orientations, and sometimes that all doesn’t go exactly right during the manufacture of the tire. I think you are on the right track to try to isolate where the sound is coming from, then you can swap tires to see if it follows the swap. Some shops have a listening gadget for problems like this, where they can put microphones at various places around the car and go for a drive, switching which microphone they are listening to.