“Plop” is the sound of something dropping into liquid and making a small splash. “Plink” is the sound of something dropping into container when it makes a high-pitched metallic sound, such as a coin into a drinking glass.
“Plop” can also mean to drop or fall inelegantly: “He was tired and just plopped [himself] onto the sofa.”
“Plink” can also mean shooting small-caliber arms at targets such as bottles or tin cans.
Perhaps that’s regional. I’ve never run into it.