The bunnies are the driver's favorite passengers

Canaan is singing "Wheels on the Bus". He was singing away, then he got to "The bunnies on the bus go..................." (long silence). 
Abrupt switch-"The horn on the bus goes toot toot toot!" Apparently, there is no noise acceptable for bunnies to make.
After the horn, he moved on to moose-"The moose on the bus go moose moose moose!" I guess moose noises are hard to pin down too.