I saw a few today and it was a very hollow ‘knocking’ sound while it’s beak was moving.
quite an impressive noise! Is there a name for this sound?
In the case of White Storks it's called bill-clattering and it's most often heard when they are on the nest. Probably some sort of bonding exercise.
I can't say I've heard it in the case of Coots although that's not to say they don't do it. However, they can make a sort of hollow knocking sound like this
Play this audio clip
Patrik Åberg, XC448651. Accessible at www.xeno-canto.org/448651
which the recorder describes as a "mud call" - no idea why.
Is that the sound you heard? If so, I suspect (although I don't know for sure) that it's done with the throat rather than the beak. I think bill-clattering would be much more rapid.
Quite a bit of speculation on my part