OK so when I get up to my normal cruising freeway speed in my 73 K20 I can hear my T-Case making noise. Between 2500-3000 RPM depending on hills and how late I am for work. I hear it coming from the shifter area. The fluid is good, clean, and plenty of it. Is this just the sound of an old gear driven T-Case?? Seems to be working fine!! Or should I start saving money for a rebuild??