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. Make sure that the muffler screws are tight first, then check the other screws on the motor. If that doesn't work, you probably have a bent crank. Have a fellow club member take a look at it. If you aren't in a club, join one. Best $25 or so you'll ever spend.