After a particular hot day with my AC running, as I got out of my 88 190e 2.3, there was the sound of liquid moving across the inside of the dash. There were no leaks, just the sound. I actually took video, which I will attempt to upload. Can anyone shed any light as to what it is?

Does this make noise when you are stationary?

This is semi normal as the expansion valve cycles but low freon levels can cause this. Is the cooling level OK or does the in car air temp track the noise?

If it makes noise only when going around a corner, there might be a condensation drain clogging up. Be careful with this water, it can be filled with mold , bacteria.

Vacuum motors can make a sound similar to flowing liquid.

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I've only heard it one time, after I shut the car off. Never when I'm driving. Cooling level seems fine, not sure what you mean by " does the in car temp track the noise" but I don't think so.

With the latest bit of info you posted. It sounds like the expansion valve opening fully after the compressor stopped. Or a vacuum noise as someone posted.

The tracking temp would be : The AC temp goes slightly high, noise occurs , temp drops as more freon hits the evaporator.

Thanks for all the great information. Is this something to be concerned about? If yes, what would be your next move?

Unless you have a leak in the interior of the cabin, I wouldn't worry about it. I have experienced this sound with other brand cars as well. You likely have a small air leak in your coolant system that is drawing in some air, causing the bubbling sounds. As long as your (heat/cooling) systems are working properly, I'm sure you have other more pressing concerns.

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If the noise is from the expansion valve opening, that is completely normal and sometime occurs on cars while the AC is in operation. Do a web search for how an AC or refrigeration system works.

My 97 SL320 and 97 C 280 make expansion valve noises while the AC is in operation. Both valves are new, the SL was changed due to it sticking shut, the C because I had the under hood duct work out to change the windshield and the valve was easily accessible. ( I don't wait for parts to fail, I change them when they are at their expected end of life. )

If the noise if from the vacuum control system as others suggested, I don't have specifics on your car. I have seen various cars have a failed vacuum check valve that allows vacuum to bleed off once the engine stops. When the system defaults to "Hot" and "Defrost" , sometimes it makes noises.

Another possibility is the brake booster developing a vacuum leak. A test would be to run the engine, shut it off then wait 60 sec or so, press the brake and see if you still have power assist for 2 or 3 presses. ( Pedal pressure will get progressively higher after each press. )