Sounded like a raven, flew like a duck

Further detail: I was in a wood and I heard a very deep crark, crark from behind and above me. I looked in the direction of the sound but couldn't see anything because of the overhead canopy.
The bird was calling every few seconds as it flew over my head (and the trees) and I followed the sound although I still couldn't see anything.
Then quite a way in front there was a small opening in the canopy and I saw the bird in silhouette from behind for about a second. It was flying very fast with rapid wingbeats, like a duck, and that's what I would have guessed it was except for the sound, which was very deep and similar a raven.
The way it was flying was not in the least like a corvid so I'm sure it wasn't a raven but some kind of duck, but I can't think of any duck (or small goose perhaps) that sounds like a raven! I say small goose because the type of call reminded me of a skein of geese flying overhead and calling to each other to maintain contact, although there was only a single bird that I saw and only one bird calling

That's all I can tell you - I just hope someone might come up with some suggestions as I'm baffled

We had both Gadwall and Tufted Duck calling in flight on a local site today and they sounded a bit croaky. We also heard and saw Raven, rare in this lowland patch, which was not that different from the ducks.