He isn't certain what the small sound from Aziraphale is, but Crowley instantly loves it and promises himself he's somehow going to make the angel replicate it.

Crowley feels that trust from Aziraphale, and it tempers his other desires----none of them as pure and wholesome as the lovely kiss they are sharing at the moment and he's trying very hard not to think about them lest he accidentally share them with Aziraphale. He slips his tongue between Aziraphale's lips delicately, giving him the option to back out of this new development the moment he wants to.

How long has he wanted to kiss the angel like this? So very long, now. Everything that fell apart in their world shattered those lines that were stopping them before, that were keeping them from kissing like this, from holding each other like the world outside doesn't exist.