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]