I hear three sounds:
The teeth grinding. I think it's called, "bruxing." I have been told that it's kind of similar to when cats purr.
Chirp. This is a high, clear sound, that is generally used conversationally, or else muttered under the breath. It sounds nothing like squeaking, and can indicate anything from "surprise" to "thinking." Gregor has said to me, "CHIRP!" like, "How do you like that?!?" When he gets busy grooming me, and gets around my ears, I hear him muttering, "chirpity chirpity chirp chirp chirp."
Squeak. The squeak is almost always "bad," at least from Gregor. I hear squeaks when the time comes for a bath. The translation is, "Don't put me in that water!" The squeak can also be conversational, however. Gregor was headed for my seashell collection (again,) and I gave him a command that he knows: "Gregor, come here!" He turned around, looked me right in the eye, and squeaked once. ("No!") After I stopped laughing, I picked him up, and told him that ~I~ tell ~him~ "no," and not the other way around.
Just my 2Â¢.