Either analog or digital will buzz when it's trying to find it's center. Analog is just a much lower frequency so it's a little harder to hear. Digitals do make a fairly audible squeal.
This is also a good way to set your endpoints. With the wheels off the ground you should be able to turn lock to lock without the servo "buzzing" at either end. If it does you need to turn down your endpoints to make sure you don't burn out your servo.