Based on my own experience, I can report that an extended "ooooo" sound is used in this way.¹
It is used in other ways, too, but in most cases it is an expression of mild mocking, as if the person making the noise is pretending to be scandalized by the possibility of the two people being together in one way or another.
There's a tone of excitement, but also maybe some judgement. A lot of all of this will depend on context, including the relationship of the speaker to the person or people being teased, audience present, tone, and facial expression. Excitement might be indicated by a shorter sound with raised pitch in the middle, while mock shock might be expressed with a longer "oooo" that either goes up in pitch at the end or builds steadily higher throughout. This last possibility can be generalized to any time someone is suggesting that someone has been "found out" whether they are being discovered to be an "item" or misbehaved in some mild way.
Physical gestures that might accompany this sound include nudging, eyebrow raises, etc.
Children might do this sort of teasing with the K-I-S-S-I-N-G song, and adults might also just to be silly.
[Name] and [Name]
sitting in a tree,
First comes love,
then comes marriage,
then comes baby
in a baby carriage!
1. I attempted to substantiate this part of my answer with website references, but online search for "ooo", "teasing", and/or "couple" has proven to be quite the NSFW minefield and I am currently at work.