It's not just LA's but any plain bearing engine tends to rattle a little at low throttle settings. There's nothing to stop the crankshaft moving backwards and forwards and they must have a small amount of movement to allow for the crankcase expanding slightly as it warms up. The rattle goes away as the revs increase because the extra thrust from the prop holds the crankshaft forward. Don't lose that steel shim washer that fits between the crankcase and the prop driver though. It sets the amount of free play but more importantly it stops the crankcase getting worn out when you use an electric starter.