niggling
adjective /ˈnɪɡlɪŋ/
  /ˈnɪɡlɪŋ/
(also less frequent niggly 
 /ˈnɪɡli/
  /ˈnɪɡli/
)- used to describe a slight feeling of worry or pain that does not go away
- She had niggling doubts about their relationship.
 - a series of niggling injuries
 
 - not important synonym petty
- niggling details