gnaw

gnaw /ˈnɑː/ verb
gnaws; gnawed; gnawing
gnaw
/ˈnɑː/
verb
gnaws; gnawed; gnawing
Britannica Dictionary definition of GNAW
1 
: to bite or chew (something) repeatedly
[+ object]
[no object]
2 
[+ object] : to make (a hole in something) by chewing

gnaw at

[phrasal verb]
gnaw at (someone)
: to be a source of worry or concern to (someone)