A while ago, Paul D. wondered about the history of words beginning with cl-. Unlike words beginning with gl-, these can be identified as onomatopoeic since they all describe sounds. Also unlike gl- words, they are not specific to Germanic, in fact they might have arisen independently in different languages.
These are from the OED except where noted.
clack from French claquer, compare Old Norse klaka "to twitter",
clang partly influenced by Latin clangere "to sound", Greek κλάζω klázō "clash, rattle"; German Klang "sound" is not related, "being an echoic word which has separately arisen in German"
clank, perhaps from Dutch klank, possibly of native origin, a combination of clink and clang
clap from Old English clæppan, clappian "to throb", compare Old Norse klappa, Old High German klapfōn "to clap" (AHD)
clash combination of clap, clack and dash, splash etc.
clatter from Old English *clatrian from PIE *gal- (AHD)
click, compare Old French clique "tick of a clock", Dutch klick "tick", "may have arisen independently in different languages. In English and Teutonic generally, it appears to stand in ablaut relation to clack, as expressing a thinner and lighter sound; cf. chip, chap, clip, clap, clink, clank."
clink compare Dutch klinken, "we cannot tell whether Middle English clinken went back with the Dutch to an Old Low German *klinkan, or was of later adoption or origination in England"
clip compare Old Frisian kleppa, Old Norse klýpa "to clip, pinch"
clip-clop "Imitations of sounds of alternating rhythm"
clock from Middle English clocke, from either Middle Dutch clocke "bell" or Norman cloke, cloque "bell". Compare German Glocke "bell". The Norman is from Medievel Latin clocca "bell", probably of imitative origin
cluck from Old English cloccian (as in the Scottish and northern dialectical clock "To make the peculiar noise of a brooding hen: to cluck"), compare Middle High German klucken, Swedish klucka (OED), Latin glōcīre, Greek κλώσσω klṓssō "to cluck" (AHD)