2014 CX9 - bought it used (CPO) from a Mazda dealer. Owned for two years with no issues (2 full NJ summers). This summer I had the water sloshing and the weird sound with left hand turns. A few searches and came up with a video on youtube showing how to suck out the drip opening with a shop vac. That was mid-July 2019. Had to do it again mid August and now, again, in mid-September. I have held the shop-vac over the hole, sealing it pretty well and gave it a whole lot of time on FULL SUCK POWER, even after all the water came out (each time I was able to suck out about 3 quarts), with the hope of clearing whatever may have been in there causing the re-clogging. But the problem keeps coming back. Is there a lasting solution???

 

How clean is your pollen filter?

Locate where the drain tube attaches to the heater box inside the vehicle,remove it and see if you can see anything inside the heater box, then find something to carefully push thru the drain tube to clear any blockages.

Spiders are common for crawling inside the tube & blocking them.