Making you worried, in a quiet way that stays with you. Something troubling is not frightening — it does not make you run. It is the piece of news you keep thinking about while you brush your teeth. 'The report contains some troubling findings.' 'I found his silence troubling.' Compare it with its neighbour troublesome, because students mix them constantly: troublesome things cause you practical bother, troubling things disturb your mind. A troublesome cough keeps you coughing; a troubling cough makes you wonder if something is wrong. The word is quite formal and belongs mostly to writing and serious speech — news reports, doctors, teachers being careful. It is often chosen exactly because it is restrained: a journalist who writes 'troubling' rather than 'shocking' is telling you this is bad while keeping a calm, measured voice. That restraint can make it hit harder, not softer. There is also the plain -ing form of the verb, as in 'What's troubling you?' — a gentle, kind way of asking someone what is wrong.