grum ‧ bling /ˈɡrʌmblɪŋ/ BrE AmE noun
1 . [uncountable] ( also grumblings [plural] ) a complaint about something:
She paid up, with some grumbling.
Soon, the grumblings turned to open discontent.
2 . [uncountable] a low continuous sound:
the grumbling of distant thunder