Finally, I'm able to get the video on here. You can hear it pretty well especially in the beginning, even above the road noise. And I've tried everyone's suggestions so far and no luck.
My car had a noise not long after I bought it in the climate/transmission panel. They did something to it and it stopped. BUT I still have a rattle in the overhead console, door panels, sunroof and rear speaker cover. All of which have been addressed, but without any resolution. I just got the car back Friday afternoon from having a new overhead console installed to help stop the rattle.
The rattle in the door panels might be caused by the control panels in them. They're just lightly held in by 4 metal tabs. I recommend you wrap the metal tabs and just under the panel's lip in nylon flock tape to deaden the sound. I can send you pictures if you don't know what I'm talking about.
My first thought would be the fan/blower. it is very easy to remove the glove box. I would remove the glove box and drive, since you have to be doing 40 mph and maybe it will give you some indication it is the blower motor. I must say if my thought is correct I do not understand why you have to get up to 40 mph for the fan to rattle.
Now it's no longer limited to just 40mph. Now I hear it at any speed, any outside/inside temperature, no rhyme or reason. It'll go quiet if I get on the gas to get the RPM's up, then it comes back when the RPM's come back to normal. I'm dying here.
If I were you, I'd take it in. Rattles are not only annoying, but hard to narrow down. I'm with you in that I like to avoid the dealership, however the car is still under warranty. It really isn't your problem to have to narrow down. If the dealership wants to play games with you, make it known that you are the customer and an unsatisfied one at that. If they're wise, they'll do whatever they can to keep you, and your rattles, quiet.
I had mine in this past week, they took the headliner off, put some felt strips down and now I don't have anymore rattles. My perception of the car has improved 100% since doing so as well!
There is a rattle in my dash also. It happened after one of those transmission recalls. My transmission failed and a new valve body put in at 18,000 miles. Got it back yesterday and still has that rattle. Dealer said he would probably have to remove dash to find it. He rode in it and heard it. Made an appointment for next week. Wish me luck.