Originally Posted by Myopic

@M60A3Driver No, the passenger footwell carpet wasn't wet. When I bought the car, I immediately bought rubber mats.
In the photo below, I've marked in red where the strange noises are coming from - in my opinion.
I briefly visited an auto electrician's workshop, who initially determined that there were various air conditioning flaps in that area, and that those might be the problem.
The electrician said he'd have to disassemble the center console and check.