I've heard of people using sticky back weatherstripping to cover the top venting louvres and that helping quite a bit, the rest of my jeep is too loud to notice anything so I ignore it.
Black weather stripping on the bottom 4 fins. It stops the air from hitting it and then hitting the top of the Jeep, which makes the whistle sound. You can get like 25 feet of stripping for like $10, which is more than enough
I just used little pieces of mud flap rubber off of a water truck. Found it along the road. Spaced them through out all the fins. No issues and was free.