Rattle is normal for all bushing engines. Some will make more noise that others. To reduce the noise as much as possible, go to a fuel that has a major part of the oil package being castor oil. Synthetic-oil fuels will allow the engine to be noisier. If your oil mix is all-castor, then the noise will diminish over time as a slight varnish will build up between the case and the crankshaft counterweight.

Don't worry about it, though. It's not going to hurt anything.