First off! This is the 2005-09 sub-forum section.. The water sloshing issues are directly related towards 2005-09 S197 models.. Being that you own a 2019 S550 model, neither your vehicle nor aftermarket sub installation has any direct relevance in common towards this thread.

That being said, your post should be submitted in either the General Chat sub-forums or 2015-20 sub-forums.. Meanwhile, welcome aboard.

 

Also just an FYI: Most of the water sloshing issues on the 2005-14 S197 models are directly related to water build-up inside the passenger side section of the firewall. Below the cabin air filter underneath the front cowl section, there's a rubber grommet with a slit on the bottom. When debris and leaves get inside the firewall thru the front cowl section, the debris reaches down to the rubber drain grommets which eventually leads to causing them to block up. Once they become blocked up, water builds-up inside the firewall with nowhere else to go.

If the water sloshing sounds are coming from the front of the vehicle? Chances are it's from water build-up underneath the front cowl section located inside the passenger side firewall..

Also keep in mind if the drain grommets remain blocked up? water will eventually make it's way inside the passenger side of the car, down into the floor thru the cabin air filter.

 

Throwing it out there, since I didn't see it mentioned. If you have water sloshing noises, generally it's one of two things: first and already mentioned, you probably have a blocked drain under your cabin filter due to plant debris building up and blocking the rubber drain hole. Take the rubber thingy out and you'll never have the problem repeat, although you may have to replace your cabin filter. However, you might have the second source of sloshing, which is the doors. There are two drains on the inside bottom of the doors which easily become blocked. They're covered with a black horizontal clip which is supposed to allow water through, but easily becomes clogged. Again, the best solution is to remove the covers. I took mine off 10 years ago and have had zero issues as a result.

 

Totally forgot about the door drains lol. Only reason I haven't removing the rubber drain grommet below the cabin air filter, is my car always stays garage kept when not being driven and never gets exposed to wet or bad weather conditions. I also check the rubber drain grommets annually while checking the cabin air filter to make certain no debris nor dirt has accumulated below the cabin filter section in the firewall.

Meanwhile, I should probably check the bottom door drains more often to make certain their not blocked up due from dirt and debris following each car wash

 

Mine had a hate - hate relationship with a Memosa tree. Got it down to 5 min or less to unblock the drain.

 

Those hate relationships with trees are always the main culprit, no question.. Thank goodness you figured out that it was the drain plugs