I bought this used car about a month ago. I was told that they changed all the fluids and made sure everything was running ok. About a week after I brought home I started to hear a noise in like my dash or somewhere around there. I only hear it when I first put my car in drive and take off. It sounds like water or some sort of liquid is moving through my dash.
The evaporator drain in the HVAC case is clogged.
Basically, the condensation that comes off the air conditioner when running collects in the plastic case that surrounds it, and then drains through a small rubber tube through the firewall or floorboard area to the ground outside.
Because this case usually collects fresh air from outside (usually the vents on the cowl, directly under the windshield outside) it also collects dust, dirt, leaves, pine needles, etc.
These clog the drain, the condesation (water) level build in the case, and this causes the liquid sloshing noise you hear. Because it's liquid, and it's sloshing...
This is a relatively simple and inexpensive repair that most any shop should be able to provide. Basically, you unblock the drain from the outside, and empty the nasty contents of the HVAC housing.