persistent, relentless, unrelenting -- (never-ceasing; "the relentless beat of the drums")
haunting, persistent -- (continually recurring to the mind; "haunting memories"; "the cathedral organ and the distant voices have a haunting beauty"- Claudia Cassidy)
persistent, lasting -- (retained; not shed; "persistent leaves remain attached past maturity"; "the persistent gills of fishes")
dogged, dour, persistent, pertinacious, tenacious, unyielding -- (stubbornly unyielding; "dogged persistence"; "dour determination"; "the most vocal and pertinacious of all the critics"; "a mind not gifted to discover truth but tenacious to hold it"- T.S.Eliot; "men tenacious of opinion")
(Sound)
