Shocking, startling, jarring, and surprising are a few synonyms.
intimidating, alarming, frightening, unnerving, disconcerting
perturbing, troubling, worrisome, distressing, upsetting, alarming, unsettling.
Scary, frightening, alarming, unsettling, disquieting, disturbing, freaky, ghostly, nightmarish
soothingly
Other words for startling are alarming, astonishing, shocking, surprising, unexpected and frightening.
terrifying, horrifying, scary, paralyzing, alarming,apalling... The synonym you should use depends on the context.
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Marie with some alarm called the Police after hearing the tree brush against the window.
It was alarming how fast he grew.
scared and worried terrifying, shocking, alarming, startling, horrifying, menacing, fearful
Alarming There is an alarming amount of cheating going on in the classroom.