WordNet (r) 3.0 (2006):

clunk
    n 1: a heavy dull sound (as made by impact of heavy objects)
         [syn: thump, thumping, clump, clunk, thud]
    v 1: make or move along with a sound as of a horse's hooves
         striking the ground [syn: clop, clump, clunk,
         plunk]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "clunk":
   bump, chink, click, clink, clop, clump, crump, dull thud, flick,
   flump, pad, pat, patter, pitapat, pitter-patter, plump, plunk, pop,
   rap, tap, thud, thump, tick, tinkle, tunk