Are they all digitals?

If the tail servo is digital it will buzz more than the others. That is normal. If the tail servo has suddenly got louder, or the noise is definately coming from the gears, then I agree with FHHuber. If it was metal geared then I might try changing the gears, but the DS811 isn't.

If you are at all concerned about the servo, I would put a new one in. I have had a servo failure recently in flight and it's not funny!