If Aztec's keel is not all the way down then it tends to swing on the cables making a slight bonk noise every so often with the swaying of the boat.
Check that all the tension on the winding mechanism goes off the wire cables and the winding handle goes slack, thus indicating the keel is at its bottom limit.
I don't believe the method of construction of the keel would allow it to bump around in the keel unless under the most violent of sea motions, and then everything in the boat would be bumping around.
The only other cause I can think of is worn keel cheek blocks where the bottom of the cheek block has been rubbing and wearing on the top of the fixed keel stub, so the keel is hanging on the cables.
Alternatively maybe the keel lifting cables have always been a bit on the short side and it has not been noticeable because of mud and debris in the bottom of the keel slot, this may have now worn and washed away.
There is one other thing I can think of, if the keel has become wedged in the slot and the winding handle has continued to be wound down allowing the cables to become too slack it may be possible for the ferrule crimped on the ends of the cable to push out of the "bronze nut". When tension is reapplied the ferrule may not return into the recess because it catches on the edge of the recess in the nut not very likely) thus shortening the strops by about 15mm.
Anyway check the handle goes slack.
Mike Edwards
Seal 28 "Aztec"
Mike Edwards
Seal 28 "Aztec"