Roland C. Ross (1924) gives the following graphic description of the croaking note:
The common call is a low-pitched, hoarse "skeuk," the lowest and heaviest voice on the flats. It struck me as a dull croak, coming pretty regularly from the feeding birds, and especially strong when they took wing. A lone bird in joining the flock would croak his coming. The sound can be imitated in quality and form but in a higher pitch. Make the facial contortions necessary to "cluck" to a horse, but don't "cluck"; make it "skeuk," and locate it in the wisdom teeth on the side being dislocated. Pitch it low; it will still be two tones too high. At a distance the sucking or harsh quality is lost. A softer, more musical rendition is given when the birds are well bunched and feeding, which came to my ear as "chook."
John T. Nichols (1920) says: "The flight note of the knot is a low-pitched whistle, frequently in two parts, with a peculiar lisp or buzz in it, _tlu tlu_."
Doctor Ekblaw describes the notes heard on the breeding grounds as follows:
Four distinct calls characterize the mating and nesting season. Most common are two piercingly shrill calls uttered generally on the wing, one of them resembling _wah-quoi_ and the other _wee-a-whit_, easily distinguished, but somewhat alike. The long-drawn-out _coo-a-hee_, or _coo-hee_, is a soft, flutelike call also given in flight, but nearly always back among the hills, far from the shore where the nests are hidden. This flutelike call appears to be a signal or recognition call. The fourth call is a sharp, querulous _whit_, _whit_, _whit_, almost like a cluck, often given singly, but more often many times repeated. When their nesting haunts are invaded or their feeding grounds disturbed this call expresses their displeasure.
_Field marks._--In spring plumage the knot is easily recognized by its reddish breast, which, however, is not as conspicuous as might be expected. In immature and winter plumage the best character is the absence of any conspicuous field mark. Even in flight it seems to be a plain gray bird; the rump and tail appear but little lighter than the rest of the upper parts and the faint white line in the wings is hardly noticeable. Its larger size will hardly distinguish it from the smaller sandpipers except by direct comparison. Its short, greenish yellow legs and its prominent bill might help one to recognize it under favorable circumstances.