Biennial rhymes with:
triennial, bicentennial, centennial, decennial, millennial, perennial, quadrennial, triennial, bicentennial, centennial, ceremonial, colonial, company'll, decennial, intracranial, lineal, matrilineal, matrimonial, menial, millennial, patrilineal, perennial, quadrennial, testimonial, triennial
Biennial sounds like:
banal, baumel, benel, biannual, biennale, binomial, biphenyl, bonelli, bonello, bonilla, bonnell, bonwell, bunnell, bunol