Looks like 'mayun' but I've no clue what that could mean :/

"howan", I think. It doesn't seem to be actual speech, but based on Mokou's reaction in the next post, I'm guessing it's either an indrawn/released breath or just a general sense of amazement.

I thought it was "horen". An SFX for someone falling in love with something.

It's the "touching a big boob" sound effect, guys.

To clarify, the text says ほわん (howan) and is a sound effect for a warm and soft object. It could be thought of as mixing ほかほか (hokahoka, warm) and ふわふわ (fuwafuwa, soft).