Fan noise and footwell water - possibly related, methinks?

[tintin]

About a week ago, my S8 developed a noise behind the dash, reminiscence of an old hairdryer on a "low" setting. The noise starts about ten seconds after starting the engine, and isn't affected by engine revs, or by the fan speed of the aircon blower. Heater still works, possibly a little less effectively, but I might be imagining that…Ideas and suggestions welcome: from memory I had the blower motor replaced about three/four years ago, can't recall if that was the same noise though - it's age finally catching up with me …..

On a possibly related point, the car decided to dump a mug full of water on the passenger carpet today on a right-hand bend, and there's a definite sloshing sound - but only, I think, when the aircon is on - and the foam under the carpet is wet, though not sodden. The drains were cleared last spring, including the orange nipple thingy being flushed with a compressor, so I'm wondering if it's just the ridiculous amount of rain, blocked drains again, or related to the fan noise?

All ideas and suggestions welcome, as ever.