Can you tell from where? Windows open or closed? With windows closed and the top in good shape, having the fan on the 3rd setting seems to quiet things down a bit at highway speed. Go through all the seams and hardware to make sure everything is seated properly and tight.
I have never personally seen them, but I think there are insulated head liners you can get, but I would make sure the original top is in good shape first.
You might be missing you mucket seals. There are two seals that are on both the driver and passenger side. They go between the frame and your upper door surrounds. I bought my jeep last November and my top was rattling like crazy, especially on rough roads. Quadratec sells them for about $20 a piece. I bought a pair of used ones from davey's jeeps for about $25.
If the mucket seals are in place, try zipping all the windows fully shut and turn the vent on with the fan up to about the #2 position. If your top is air tight, that will help "inflate" the top which usually makes it dramatically quieter. And running the a/c in its recirculate mode will usually make the top louder since it places no air pressure against the fabric.
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