Alright, so I've done some experimenting over the past few days. At first, I took a piece of cardboard and taped it to the front of the light bar, simulating a wind deflector. You can imagine the looks I got while driving it around. The important thing was that it did help a little. Started a few miles per hour later, and maybe half volume at highway speeds.
Home Depot sells large sheets of clear Lexan plastic, but something long enough to fit across the width of the Jeep was going to be $60. Not something I felt like paying, for what seemed to be only a bandaid.
While I was out, I realized I had a large air compressor with a blower nozzle, and wondered if that would let me reproduce the whistle at home, in the driveway. It took some playing, but after blowing air around multiple parts of a light, I heard a faint whistle. Seemed to be coming from the bottom two corners of the light rings.
Begrudgingly, I removed one of the light rings and tried the air gun again. If anything, the whistle seemed louder, but it was definitely coming from the bottom and sides of the lamps.
Since those 50" LED lightbars have whistling noises coming from their cooling fins, I thought maybe the two would have the same fix. I took some rubber sealant, and filled in the gaps between the rubber rings on the front and both sides of the base, and let it sit for a few minutes before trying the air gun again.
The noise was still there. No difference whatsoever. At this point, I was confused. There was no noise before, with just the light bar, and even after adding the 6" wide mounting plate to the bar, there it was still silent.
I took the blower to the light again, moving it around to get the loudest sound. Then, I started blocking the air with my hand, moving it closer and closer to the light, until I found the source...
On the underside of each light are four screw holes, two on each side. Covering one with my thumb, the whistle stopped. Remove my thumb, and the whistle returned.
I took to removing the sealant, and ended up removing most of the PlastiDip too. Really is amazing how it peels right off.
Filled in all four holes on each light with that same rubber sealant. Would've been easier to do with the lights off the Jeep, but I didn't feel like redoing all the wiring. Besides, it's only gravity.
After each hole was sealed, I waited a little and started to re-dip the light. Four or five coats later, the lights all looked like the did before. But did it work?
I started up the jeep, and pulled out of the driveway. Seemed silent driving through the neighborhood, which was an improvement over the faint beginnings of the whistle I had heard before. But, maybe the winds were at my back. Merging onto the highway, as the speedometer swept past 40 and towards 50, I noticed it was still quiet. As I approached 60, there was no whistle, only a turbulent wind noise. At 70, the same, only a little louder. I can live with the wind noise. It's only a little more noticeable than the standard cloth-top noise the Jeep's always had. I turned around and headed home, my lights fixed. Best of all, I didn't spend the $60 for the plastic!