Biannual rhymes with:
annual, asexual, audiovisual, bisexual, conceptual, consensual, continual, contractual, emanuel, emmanuel, eventual, factual, gradual, habitual, ineffectual, intellectual, manual, massmutual, menstrual, parimutuel, pascual, perpetual, punctual, residual, ritual, sexual, transsexual, virtual
Biannual sounds like:
banal, baumel, benel, biennale, biennial, binomial, biphenyl, bonelli, bonello, bonilla, bonnell, bonwell, bunnell, bunol