Untruth rhymes with:
truth, fruth, rueth, ruth, truth, bluth, booth, buth, duluth, fruth, gluth, guth, huth, kluth, knuth, lueth, luth, meuth, muth, pluth, puth, rueth, ruth, schuth, sleuth, tooth, truth, uncouth, vermouth, wermuth, youth
Untruth sounds like:
underrate, underrated, undertow, underwood, underwrite, underwrote, undeterred, unhindered, unitrode, untoward, untreated, untried