Slight Servo Problem

I'm having a slight problem with my Z axis servo. Everything is
running fine but it oscilates (buzzes) MUCH more than the other 2.
Its also running hotter, over 50 degrees C when the other two
actually feel cool to the touch.

The servos are all Electrocraft E543's with renco encoders.

The Z axis is almost always buzzing. And at a much higher frequency
than the other two. The Gecko 340s are all set exactly the same.
Factory settings on Gain/damening and current limit set to about
half. I've tried playing with the gain/dampening knobs, even to the
point of having gain at the minimium and it still seems to buzz much
the same way.

And the problem is not related to the constant Z axis weight. The
machine isnt using ball screws and the z axis probably only weights
20 lbs anyway (mini mill). And i've tried moving the handwheels
slighly to move it past the neutral position but it still buzzes.

One other weird thing I noticed is that the other motors are
perfectly silent when running. No noise at all, and just some very
slight noise when stopped as any servo has. The Z servo, on the
other hand, makes the same "stepping sound" as a stepper during
average cutting speeds (~10IPM).

I cant get the same speed out of the Z axis either. It oscilates
heavily and even causes the drive to fault before 3000 RPM when the
others can go to 4000 no problem.

It is doing the job now, no lost steps or other problems, but I'd
like to have it working as the others do.

So any advice? I havent had a chance to swap drives yet and isolate
that. Is that the next step or can you guys diagnose it from what i
mentioned above? Thanks in advance.

Mark