Ok...here's what I've noticed some time ago and has left me baffled about, since.
I've driven the car under normal rainy weather often but, in mid-December, the weather was really bad around here, on account of a heavy storm.
I was driving under a very heavy downpour when I noticed that, at about 80km/h and above on the freeway, there was this muffled low-tone "plunk, plunk, plunk" noise in the car.
It sounded as if very large droplets were hitting the car (...and that would have been my guess...) if it wasn't for the fact that the noise came from all around the car - top, side, underneath...literally everywhere.
I couldn't figure out what it was and the only lead I have is my wife (...who worked for a high-profile Japanese car manufacturer...) told that, once, a specific imported series of a particular model from that brand was manufactured with the noise-supression coat missing in the wheel arches and they had to find an improptu solution by appliying the coating here, in Portugal, after the import.
It is possible that something similar can could be the source for what I've heard, as a significant amount of water sprayed from the wheels and hitting an uncoated wheel arch could make the arch work as a resonating chamber, propagating the sound to make it seem as coming from everywhere.
There's been nothing but normal rain around here since, so I was not able to check for this problem again.
One of these days I will be dropping by my dealer for an inquiry about other inputs they might have on this subject.
I will keep you posted.
WV