Example : The relentless beat of the drums.

Synonyms : relentless, unrelenting

Example : Haunting memories.
The cathedral organ and the distant voices have a haunting beauty.

Synonyms : haunting

Example : Persistent leaves remain attached past maturity.
The persistent gills of fishes.

Synonyms : lasting

Example : 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.
Men tenacious of opinion.

Synonyms : dogged, dour, pertinacious, tenacious, unyielding