String Trimmers

I just purchased a 223L trimmer and it starts super easy! However when it is idling it vibrates like crazy until I pull the throttle, then it lets up. I bought the same product earlier today and returned it because it was doing the same thing, so I thought it was defective and exchanged it only to find that this one is doing the same thing! Is this normal? I thought for $250 it should run a lot smoother, please help!!