My 2016 Fiat 500, which I just love to drive despite its various peccadillos, developed a nasty buzzing rattle coming from the dashboard at about 2800 RPM. I had asked Mrs R to put her hand on the defroster vent once, and that stopped it, so the vent was my chief suspect.
The defroster vent is a wide, slender nozzle that snaps onto the top of the dashboard to direct hot air onto the windshield. I’d pry it out and glue it into place, except that access is tight because of the slope of the windshield, and the word on the street (“Johnny, what’s the word on the street?” followed by the rustle of bills changing hands. <== obligatory “Police Squad” reference) is that the clips holding it are fragile and tend to break. So what I did was, I cut a few little squares of sticky-backed black felt, and shoved them under the edge of the vent using a putty knife.
I had done this once before, maybe a year or two ago, resulting in a temporary reduction of the buzz; but it came back after a while, probably because the felt slid out from between the vent and the dash top.
So I did it again last night, making less of a mess than the first time, and on a short trip to the local cafe for cocktails at lunchtime (being retired is great), the dashboard was silent as a tomb. Hopefully it’ll last this time.
If it doesn’t, I’ll just wait until I have to have the windshield replaced, and then, with the improved access that having the windshield out will give, I’ll carefully pry out the vent and then glue it back in using something with good damping characteristics, like silicone sealant or that insanely tenacious black glue they use to set windshields (I think it’s polysulfone).
But, at least for the moment, I’m feeling pretty good about solving a car problem. 
