Jamieson, Fausset and Brown on Zephaniah 3:17

17. he will rest in his love--content with it as His supreme delight (compare Lu 15:7, 10) [Calvin], (Is 62:5; 65:19). Or, He shall be silent, namely as to thy faults, not imputing them to thee [Maurer] (Psa 32:2; Eze 33:16). I prefer explaining it of that calm silent joy in the possession of the object of one's love, too great for words to express: just as God after the six days of creation rested with silent satisfaction in His work, for "behold it was very good" (Ge 1:31; 2:2). So the parallel clause by contrast expresses the joy, not kept silent as this, but uttered in "singing."