Just a thought but have you ever dropped or moved you fuel tank? I looked for a very annoying squeak for over a year. Couldn't be sure where it came from. Looked at my maintenance log when I replaced a fuel sending unit and thought I'd take a look at the tank position. Sure enough moving the tank fully forward did the trick. Part of the tank was rubbing on the rear cab panel and sending noise everywhere.
Hold the gear selector up slightly and if that stops it lube the shift linkage