ORIGINAL: ksubigbuck
My first trainer had an la 40 as well. I believe that mine had a little play in the crank too, but it shouldn't rattle. If that doesn't work, you probably have a bent crank.
I know this fellow was trying to be helpfull, but I would not worry about your crank being bent unless it developed clanking noise after it was plowed into the ground nose first at 50 mph. Even in those cases I have seen these engines picked out of the ground with a screwdriver, cleaned out, and put right back into service. So unless the engine has suffered some severe crash and developed a noise directly after, then more than likely it is fine and the noise you hear is normal!