Never worked on Mini Coopers but it sounds like your condensation line that drains through the firewall is plugged up and just needs to be cleared out......

If the condensation line plugs up, water will just build up in this line that's usually behind the dash.....and slosh around.....eventually it will back up and overflow into the interior of the car.
It's not a hard fix at all.

Check to see if your carpets are wet under the dash.

 

I have had this same noise since I bought the car new in 2008. It sounds like water rushing behind your dash. I have since installed a water temp sender in my coolant line for a gauge and then tried to bleed the coolant system of as much air as possible. Since then, I have not had the noise for over 4,000 miles. It used to occur daily, usually when I started to accelerate from a stop after the car had been sitting for a while.

All I did to get the air out was let my car idle with the coolant expansion tank cap off, and my heater on low. A lot of air was bubbling up in the expansion tank. I had read previously that some thought it may have been air in the coolant line and it make sense to me considering I have not had the noise occur anymore, but then again, I'm not a mechanic.

 

Hmm, but simply having air in my lines wouldn't explain my losing coolant....

I have to have a leak somewhere.

 

I had to top off my coolant quite a few times before I installed my gauges. I would run around, then park and notice it was below the MIN line. I'd top it off, run around the next day, check it, and it would be low again. I looked everywhere for leaks, and brought it in to the dealership. They found nothing and told me the level will vary.

Since I installed my gauge and let an much air bubble out as possible, after driving and letting the engine cool down, my coolant level in the expansion tank has stayed exactly the same, day after day. Can't explain it but having let the air out as best I knew how has solved both of my probs with the coolant level and noise.

 

Guys,
I had the slushing sound in my 06 JCW, just started a few weeks ago. Just fixed it last night by replacing the thermostat and housing. I too had topped off the reservoir with blue stuff to no avail. It is not a bad fix but a heated garage is a must. I did it in my garage and it was 18 last evening. By the way someone has a post out there indicating you may need to remove the supercharger....... not true. It is all accessed from the top by the airbox. Good luck

 

I have an appointment Friday for this problem. Coolant level is more than 1/4" below MIN.

Chef was inspected @ the dealer two weeks ago ?

I will update when applicable.

Oh - and i dont have a JCW, i have a stock S.

 

Same experience as previous posters with my '06 MCS. Sloshing sound was caused by air in the system due to loss of coolant. Sloshing disappeared after the system was bled and refilled, but the coolant leak still needs to be diagnosed. Car is at the shop so they can figure out where the leak is coming from. Bottom line: getting the air out of the system will solve the sloshing problem but you still need to find and fix the coolant leak if you have one.

 

im wondering if this thread would be better suited in the Problems forum ?

 

so i got Chef back from service today. they ran a pressure test and found no noticable problem. they bleed the lines and topped off the fluid level. the noise is gone, for now.......