The Collaborative International Dictionary of English v.0.48:

echoic \echoic\ adj.
   1. (linguistics) Formed in imitation of a natural sound; --
      of words. Contrasted to nonechoic.

   Syn: imitative, onomatopoeic, onomatopoeical, onomatopoetic.
        [WordNet 1.5]

   2. Like or characteristic of an echo.

   Syn: echolike.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

echoic
    adj 1: (of words) formed in imitation of a natural sound;
           "onomatopoeic words are imitative of noises"; "it was
           independently developed in more than one place as an
           onomatopoetic term"- Harry Hoijer [syn: echoic,
           imitative, onomatopoeic, onomatopoeical,
           onomatopoetic] [ant: nonechoic]
    2: like or characteristic of an echo [syn: echoic, echolike]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "echoic":
   answering, antiphonal, apish, battological, booming, confutative,
   conjugate, delineatory, depictive, derivative, duplicative,
   echoing, embodying, emulative, etymologic, figurative, graphic,
   growling, ideographic, illustrational, illustrative, imitative,
   incarnating, iterative, lexical, lexicographic, lexicologic,
   lexigraphic, limning, lingering, mimetic, mimic, mimish, onomastic,
   onomatologic, onomatopoeic, onomatopoetic, paronymic, paronymous,
   parrotlike, persistent, personifying, pictographic, pictorial,
   portraying, reboant, rebounding, recapitulative, redundant,
   reduplicative, reechoing, refutative, reiterant, reiterative,
   rejoining, repeating, repercussive, repetitional, repetitionary,
   repetitious, repetitive, replying, representational,
   representative, representing, resounding, respondent, responding,
   responsive, returning, reverberant, reverberating, reverberatory,
   rumbling, simulative, sounding, symbolizing, tautological,
   tautologous, thundering, typifying, undamped, vivid