Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
unobtrusive [ʌnəbˈtruːsɪv] adj (person) ненавя́зчивый/ненавя́зчив (engine) бесшу́мный*бесшу́мен
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.