When restricting ourselves to the English phoneme repertoire, this leaves us with fff, hhh, sss, shh and thh (IPA: f, h, s, ʃ, θ). Of these, hhh is confusable with abnormal breathing and (in my personal opinion) fff, sss and thh stand out more against ambient sound than shh, which is acustically similar to white noise. More @Wikipedia
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