Found another Rivian post that recommended Rhino Rack Vortex generating strips to plug the crossbar grooves. Seemed to fix the whistle for their ski racks. I think I’ll try these first.
I'm in the same boat (1 week after getting PPF and ceramic coat). From what I've found, access from the bed will only get you to the level above the wheel. To get access behind the wheel, the bumper and sensors need to come off...