Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
as -sībĭlo (ads-), āre, v. n. and a., to hiss, murmur, whisper at or to a thing (only in the post-Aug. poets): alno adsibilat alnus, Claud. Nupt. Hon. et Mar. 68; id. Rapt. Pros. 2, 225: moto adsibilat aëre ventus, Aus. Mos. 258.
As verb act.: serpens animam adsibilat aris, i. e. sibilando amittit, Stat. Th. 5, 578.