confide
His voice softens, taking on a confiding tone.
She allowed a confiding note to enter her voice.
Anxious to appear friendly, she put out her hand with a confiding gesture.

SMART Vocabulary: related words and phrases

Honesty, openness & sincerity
above board
anti-corruption
artless
artlessly
artlessness
earnest
honestly
in all honesty/seriousness/truthfulnessidiom
in good faithidiom
integrity
joking apart/asideidiom
let's face itidiom
sincerely
stainless
straight arrowidiom
straight upidiom
straightforward
the straight and narrowidiom
transparency
upright