When engine is idle and the clutch is not depressed, you can hear a slight metallic whine which sounds to be coming from the bottom of the chassis near the transmission, and when the clutch is depressed, the whine slowly decreases in volume and pitch, then disappears.
Once the truck starts rolling, the whine increases in volume and the pitch gets higher with speed, through all gears EXCEPT 4th. I have read that this is due to 4th gear having a 1:1 ratio, but I'm not sure how that pertains to my problem yet.
The truck drives fine for the most part. 1st and reverse gears are somewhat sticky at times, hard to engage, sometimes grinding when attempting to throw into 1st or reverse, but other than that there's no serious grinding or problems shifting. (This may point to a problem with the slave cylinder, I'm guessing.)
It's basically an issue of a very loud metallic whine while the truck is in gear, whether driving down the street or on the interstate. You can hear it coming from a mile away. It's been this way for almost 5-6 years now. 2 mechanics, one of whom used to own a Ranger, told me not to worry about it, but I'm sick of hearing the noise.
I have tried draining and refilling the transmission, but this had no effect on the noise. The first time I did it, it was about 6 years ago, and I believe the fluid was almost black and I thought I saw metal particles on the drain plug. So naturally I thought something was wrong with the tranny and new ATF by itself wasn't going to fix it, although I have drained and re-filled it several times since. And I did not have the means back then to just haul it to a transmission shop and ask them to fix whatever's wrong with it. Especially since it's driven relatively fine for the past 5-6 years.
I have also read that these trannys are notorious for leaking from the top, which causes a loss in fluid, which causes loss of tranny. And when I drained it the first time, I noticed that there wasn't much fluid, and what remained looked really bad.
More recently, I thought maybe the synchros might be shot but then why the noise when it's in neutral, and when the clutch pedal is not depressed? Also I could never figure out why the noise seemed to disappear when in 4th gear, which is the only gear that's quiet (it's hard to tell due to road noise but at the very least it's nowhere near as loud as the other gears.) I do understand you have some whine in reverse by nature.
I do plan on getting a transmission jack and inspecting the drivetrain components, now that I have the time and the means, but I wanted to get some ideas on what could be causing this noise given the above facts, hoping to narrow it down. I am hoping it's something simple inbetween the flywheel and the input shaft. The clutch components may be torn to hell for all I know, in addition to possibly needing a new or rebuilt tranny.
Adam