The Collaborative International Dictionary of English v.0.48:

Rumbling \Rum"bling\,
   a. & n. from Rumble, v. i.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rumbling
    adj 1: continuous full and low-pitched throbbing sound; "the
           rumbling rolling sound of thunder" [syn: grumbling,
           rumbling]
    n 1: a loud low dull continuous noise; "they heard the rumbling
         of thunder" [syn: rumble, rumbling, grumble,
         grumbling]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "rumbling":
   boom, booming, cannonading, echo, echoic, echoing, fulminating,
   growl, growling, grumble, grumbling, lingering, pealing,
   persistent, reboant, reboation, rebound, rebounding, reecho,
   reechoing, repercussive, resound, resounding, reverberant,
   reverberating, reverberation, reverberatory, roaring, rolling,
   rumble, sounding, thunder, thundering, thunderlike, thunderous,
   thundery, tonitruant, tonitruous, undamped, volleying