There is something that rattles in the dash and it does sound like the vents or more likely the ducting vibrating deep in the dash (for the windscreen demister vent?)
We'd had two Pandas do it and they went back under warranty to be sorted.
I never did find out what rattled, but I did see one being repaired at the dealers.
It was a complete dash out job and strip down of all the parts.
I did think at the time I'm glad I never tried to fix it myself, it looked a right mess.
I noticed a few foam type sticky pads left in the foot wells when I picked one up after fix, I guess this is what they used to hold the ducting in place.
On one of them, if I pushed the radio inwards, the rattle stopped, this one came back with the radio no longer quite flush to the dash panel after the fix, so was under the impression is wasn't the same rattle as the other one, though it was just as annoying.
Since then I've had a few Pandas on hire on holidays and most have rattled as well, some quite badly.