b.on the sing: (of a kettle) singing. Cf. singv.1 6a.
ΘΚΠ
the world > physical sensation > hearing and noise > degree, kind, or quality of sound > resonance or sonority > other resonant sounds > [adverb] > of fermenting or heated liquid
on the sing1927
1927 W. DeepingKitty xxx. 384 ‘All the kettles—.’..‘Two are boiling, miss; the other's on the sing.’