Here's a selection of loon calls.

https://loon.org/the-call-of-the-loon/

I'd say the fourth one, the cooing, is closest to a mourning dove. The long wail has the mournful quality but is much louder. The loony-laughing tremolo is a whole different thing.

Here's a quick coo from a mourning dove.

https://m.youtube.com/watch?v=AOZmkZ72ISI

It's also got a bit of the wing sound, which to me always sounds like they're panicking about whether they're really able to fly.

I agree that they're both different from twittering or tweeting birds in a similar way.