One explanation for why it seems to stop when you turn on the blower is that you are making the air pressure differential between inside cabin and outside more equal. At higher speeds outside air is trying to force its way into your cabin through any small opening it can find. When you turn on the blower you are pressurizing the cabin somewhat and outside air is not as determined to get inside (for lack of a better way of saying it.)
You might have a leaky weatherstrip seal. Likely culprit is the windshield seal. I've found these leaks by placing duct tape over the suspected areas, driving the vehicle at speeds that produced the wind noise and seeing if the noise went away. If the noise went away or lessened, great, you've isolated the general area of the leak. Start removing small lengths of the duct tape, say six inches or so, from one section of the tape. Drive the truck again, checking for wind noise. If wind noise is still not happening, remove another six inches of tape and repeat the wind noise test. Eventually you will remove a section of the tape, the wind noise returns, and you've found the area where the air leak is occurring.
If you put duct tape around the windshield weatherstripping, drove the truck and didn't notice any change in wind noise, the leak is probably somewhere else. I'd try duct taping around the passenger window weatherstripping next. Repeat the driving and listening process.
After going through this process, the upside is you might find the leak and have some direct knowledge of the problem area. Always good info to have for when you go to the dealer. Keep that info to yourself (don't bias their troubleshooting efforts in any way) and let them try to find the problem. If they come up with leaking weatherstripping or some other reason for the noise and fix it, that's great. If they say they can't duplicate the problem or "it's normal, they all sound like that", then you can tell them about your duct tape experiment.
The downside is you look like a dork driving around a brand new truck with duct tape plastered on the window! Luckily, it's only for an hour or so. And you could do it at night. LOL!
Good luck.
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