[78 Chevyrado]

Unhook the guage and drive around with it unhooked, then if it squeaks you will know it's the cable. If nothing, it's the guage. Sounds like it's the guage from what you say. I have seen new cables squeal because it was kinked somewhere or bent just a bit too much somewhere else. See if you can lube the guage. I don't know how, as for some reason i've never had a noisy speedo or cable, so I never had to mess with it.