ORIGINAL: Bax
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.
Take Bax's advice here, and just run the thing. IT IS NORMAL FOR THEM TO RATTLE SOME! If you send it off to Hobby services they will just send it back to you because it is typical for a bushing engine to rattle some especially at lower RPM's. If you are running Full synthetic fuel and the rattle bothers you, then also do as Bax suggest and buy some fuel with Castor, or I would recommend instead of wasting the fuel you have to just add about 2 fluid ounces of 100% Castor per gallon of fuel. Bakers Castor is fine, or you can order a bottle of Klotz to add to your syn fuel. The engine will quiet down with the castor in the fuel over time, but you will trade off this benefit by having a tarnished looking engine, and more mess on the Plane itself to clean.