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