Water Swishing Around

I have a 2000 Blazer. The sound of water moving around coming from as best as I can tell, the front dash area where the radio is installed. Where can water accumulate to the point I can hear it moving when I make a turn ? thx

If the drain for the HVAC case is clogged, rain water and condensate from the AC can fill the case.

Low antifreeze levels can also make water swooshing noises in the heatercore as well.

How does one unclog the case ? I can check antifreeze level to check that theory . thx all !