inner voice; voice of one's heart; what one really thinks
Refers to one's true thoughts or feelings, often contrasted with spoken words. Common in self-reflection or when someone's real opinion is revealed.
Contrary to his words, his inner voice was different.
I decided my career path by following my inner voice.
Compound of 心 (こころ, 'heart, mind') + の (possessive particle) + 声 (こえ, 'voice'). Literally 'voice of the heart'. The phrase is transparent and modern; no special historical derivation.