@DragoZero
I tend to agree with Kanon's explanation on "Chu"
Spoiler for spoiler:
it's probably the sound of suc**** something XD...besides why would Momo do "kissing" in that situation?? XD
Well, either way it's left to everyone's imagination I guess (hold on, why ppl would imagine it...so indecent!!! (yui's tone)