I have this "problem" as well. What the world is actually doing is trying to load all of the chunks (most of which are very detailed) and the Minecraft server can't keep up. So a gap forms in the world.
That sound when you hit the floor is the server restarting itself so that you spawn on the surface.
This glitch doesn't affect your account or your world.
This is what the original glitch looked like.
This is what the glitch looks like now:
If you receive an error message from java it means that the world you were trying to connect to has a bad connection. To fix it you could restart the server if it is your own. Otherwise if you are playing on a laptop and you are playing wirelessly you should move closer to your router. Increasing the speed of your internet usually fixes the problem.
1st picture: davidjio.com 2nd picture: deviantart.net