As tufty said, have a look under the pollen filter and you may see water pooling in the plastic housing underneath.
I could also hear water sloshing around behind the glovebox when mine had this issue.
If so, remove wipers, remove black plastic panel that fits against the windscreen, then give the back of the panel and the area that it fits to the car a really good clean. There is a channel that had dirt in it on mine, and once that was cleaned, the trim fitted flush again, and no more water issues.