elusive -- (difficult to describe; "a haunting elusive odor")
elusive -- (skillful at eluding capture; "a cabal of conspirators, each more elusive than the archterrorist"- David Kline)
elusive, subtle -- (difficult to detect or grasp by the mind or analyze; "his whole attitude had undergone a subtle change"; "a subtle difference"; "that elusive thing the soul")
baffling, elusive, knotty, problematic, problematical, tough -- (making great mental demands; hard to comprehend or solve or believe; "a baffling problem"; "I faced the knotty problem of what to have for breakfast"; "a problematic situation at home")
(Sound)
