These dashboard/windshield noises and/or vibrations have generated several threads over the years. I think the first and easiest thing to check is what 'coldsteel' has suggested. My "D" had a lesser vibrating noise as I drove on the highway than yours. After reading one of those threads I elected to tighten the hood bolts down a bit and put a black zip tie around the water line along the hood hinge. The problem was cured in less than 5 minutes and has stayed that way for 4 years. Good Luck

 

I have a 2015 Dodge Caravan and I noticed the same thing. I drove over a rough patch of terrain, had the noise and then I pressed up hard on the windshield, the noise immediately stopped. I let go and it kept going again. I think my windshield itself is loose. I just read on autolemon.com a very recent recall for a very limited number of vehicles where the windshields were attached with expired product and they can detach, primarily in an accident.

 

You can always check for open recalls on your vehicle here: http://recalls.mopar.com/

Like with the OP, I would recommend having your dealership check this out for you. Are you still under warranty from a mileage standpoint?

 

I have the very same issue starting at speed 140 Km/h (87 mph). It seems to be related to the airflow valve that is used for the recirculation. When recirculation is off the noise starts at 130 Km/h, with recirculation at about 140 km/h. This on a very smooth highway, no vibration from the road. This dash-noise has come up recently, I suspect it is also related to the external temperature, which would prompt some materials to expand.

 

I have the same thing. Only starts above 80 mph. Especially when the wind is blowing from right to left. I can't figure out what is vibrating. I've tried securing the washer lines. No help.

It seems to come from the front drivers side area. Near the firewall I'd guess.

 

I have recently seen this issue suddenly disappear, after two changes took place in my DD.
First I have installed new tires.
Then,few days later I have had a new windscreen installed, because a flying stone on the road broke my windscreen.
After those two changes I noticed that I could drive up to 210 Km/h (max speed) without that annoying rattle to manifest itself anymore.

Both things may explain the change.
When new tires are installed they get balanced and with a different balancing the resonance point of the car as a system changes.
When a new windscreen is installed the dashboard and everything around the windscreen is probably loosened and tighten again.

But my feeling is that the latter is most probably the one that did the trick here ...
Talk to a professional about the activities that entail the swap of a windscreen and check which of those might be of help in this case.

 

I know this is an old post, but perhaps this might helps someone else. The noise is actually coming from the seal at the bottom of the windshield on the exterior where the bottom of the windshield joins the cowl. Look and see if you see bubbling or space on the rubber portion where if meets the bottom of the windshield. If you do, what happens is that at higher speeds, wind catches and flaps the rubber piece making the vibration sound! A quick test to be sure this is the problem, take duck tape and seal the rubber piece to the bottom of the windshield on the outside of course, drive at highway speeds and see if you hear the noise, this is what happened to my DD. The replacement part for mine was part #5182164AH which was a little pricey but if you're handy you can get it online a little less expensive.