A couple of nights ago I recorded this sequence of an American Bittern and Black-crowned Night-Heron calling together as they flew over my house (2.04am, early Apr, PA).
bittern on left, night-heron on right
The night-heron can give very low calls, even lower than the bittern, but there's generally a rising element to them.
These calls from the Macaulay Library were at a breeding colony and are not typical nocturnal flight calls.
As always, nocturnal call id's are presumptive as the birds were not visible.