This's a vocal sound designed / used by Joel Valentine. It's also used by Hacienda Post too and may be available on The Producer's Sound Effects Library.
- Big City Greens (Heard once in "People Watching")
- Chowder (Heard once in "The Froggy Apple Crumple Thumpkin"
- Dexter's Laboratory (Seasons 1-4 out of 6)
- The Powerpuff Girls (Heard once in "Pet Feud" and twice in "Save the Date")
- The Powerpuff Girls (2016 TV Series) (Heard twice in "Save the Date")
- Samurai Jack
The sound to reference is a simple, quiet, deep "glug"/"gloop" sound. The best sources include "Dexter's Lab: A Story", "Pet Feud" and "Save the Date" (PPG), and Samurai Jack EPISODE XXVII (a number of times near the end of the episode).