Still small voice

голос совести [Этимология: библ. I Kings XIX, 12] Например: He was a man with a conscience, and... that still, small voice of his had grown louder and he had decided to make a clean breast of everything. (E. Wallace, Mr. J. G. Reeder Returns, The Shadow Man) — Угрызения совести становились все сильнее, и Рейгет решил во всем признаться.

Англо-русский фразеологический словарь