worry at something   
phrasal verb with worry verb uk/ˈwʌr.i/ us/ˈwɝː.i/
  (OBJECT)      
  反复逗弄
to shake, pull at, or touch something repeatedly
  The dog was worrying at its bone. 狗在反复撕咬着骨头。 
   (PROBLEM)      
 mainly UK不断努力;反复尝试 
to keep trying to find a way of solving a problem
  She'll worry at those figures until she's sure they've been done correctly. 她会反复验算那些数字,直到确定计算结果无误为止。