NOISY cory cats sucking air from the surface?

[hbeth82]

I've had 5 cory cats (false julii & pepper) in my 55 for a little over two weeks and in the last few days, have noticed something kind of weird. Randomly between 12am-2am, I hear one or two of them up at the surface & it sounds like their blowing kisses, kind of a weird popping / sucking noise (my boyfriend & I have spent a lot of time mocking them). It reminds me of listening to one of my gourami eat freeze-dried bloodworms. Whichever is doing it will usually swim the length of the tank, spending most of his time just below the output from either of the filters. Water parameters are good (0 ammonia & nitrite, nitrate <20). Is it just them finding food that the other fish have missed, & they just happen to find it as I'm trying to sleep? Also seemed strange that they've only begun doing this recently. . .or does it mean that they are beginning secure enough that they can venture to the surface?

Thanks!