Throughout the entirety of the movie, you never hear it. Then after he gets bashed in the head, you hear a very pronounced sound when his camera/eye adjusts its position. Should we assume that while not exposed, the skull of the T-800 has sound-proof insulation to prevent such a robotic (cybernetic) sound being audible?
The answer is too obvious actually. We know that the T-800 has human skin and tissue outside as seen in the scene where T-800 cuts his hand and by the way he describes himself to John Connor. The thickness of all this layers of tissues and muscles will act as a sound proofing mechanism. So once this outer protective layer is destroyed by T-1000, the sounds become audible.
In expansion of the idea of sound insulation by tissues, there is a very strong acoustical impedance between a liquid (or a mostly-liquid substance, like tissue) and air. There is much research in this area, with an example at http://www.ncbi.nlm.nih.gov/pubmed/18396996. But in short, you don't hear the Terminator's joint actuators for the same reason you don't hear an underwater explosion when you're above the surface, even if it is strong enough to stun or kill sea life. The exception to this is if the explosion actually reaches the surface in the form of bubbles of superheated steam, and I'm pretty sure that Terminator mechanisms aren't supposed to get THAT hot.