Source : Webster's Revised Unabridged Dictionary (1913)

Rumbling \Rum"bling\,
   a. & n. from {Rumble}, v. i.

Source : WordNet®

rumbling
     adj : continuous full and low-pitched throbbing sound; "the
           rumbling rolling sound of thunder" [syn: {grumbling}]
     n : a loud low dull continuous noise; "they heard the rumbling
         of thunder" [syn: {rumble}, {grumble}, {grumbling}]