Do they buzz if you take the pushrods off them?

If the buzzing goes away when you take the pushrod off, then you have friction in the linkage, and the servo is working to try to get to center.

If the buzzing is there even with out the pushrod connected to the servo, and the servo works fine, I generally wouldn't worry about it. You might try working the servo with the pushrod disconnected, and fighting it a little with your fingers. You should hear the buzzing get louder when you resist with your hands. (don't over do this, btw. A little bit won't hurt the servo, but if you over do it, you can strip gears and cause other problems).