It was a godless sound; one of those low-keyed, insidious outrages of Nature which are not meant to be. To call it a dull wail, a doom-dragged whine, or a hopeless howl of chorused anguish and stricken flesh without mind would be to miss its most quintessential loathsomeness and soul-sickening overtones.
Howard Phillips Lovecraft has often been accused of merely heaping together adjectives and adverbs in a vain effort to describe horrors as ultimately "indescribable". #Lovecraft himself had a low opinion of his work and thus he's full of praise for other writers who evoke the kind of weirdness he wanted, that he could feel keenly and strive to depict himself...but with such inadequacy.
Hence, in his defence, I would like to point out the difficulty of the task he's facing in this one short passage: to describe the sounds that might be made by a creature that is not an animal, nothing familiar, but a concrescence of unprecedented human magic or science—the distinction matters almost nothing here—that is indubitably alive but not by any means known to human experience.
A malformed, imperfect de novo creation, one of Joseph Curwen's less successful experiences in restoring life to the "essential saltes" of dead beings perhaps...trapped in a pit somewhere in the dark, existing only because its creator hadn't yet found a good enough pretext for destroying it.
What noises would it make, in its solitude? Howard tries his best to relate this.