1st Gen R53 Cooper S Most liked posts in thread: What is the water swishing sound coming from passenger side floor?

It's probably a clogged condensate drain for your AC sytem. Try putting some towels in the passenger's foot well, and blast some compressed air in the rubber hose that sticks out in the exhaust tunnel to the passenger's side of the catalytic converter. Full Disclosure: I've never actually done this, but it should work (famous last words).

 

When you're running your A/C is there water dripping under your car? If not, your condenser drain is plugged. You could also have a heater core leaking. Have you checked your coolant level lately?

 

Are you sure the sound isn't appearing to come from behind the dash panel? If so, it's a very common symptom of an improperly bled cooling system. I had this happen twice on my 1st Gen, even though the corrective maintenance had been done at the dealership.

If you've had a recent flush and fill, thermostat or water pump replacement, etc. you may just need to bleed the air out of the system correctly.