Hey guys I just drove my 1986.5 Nissan Hardbody pickup 5spd w/258k miles on it (I got it for free at 255k miles) about 250 miles from Ellensburg, WA to Portland, OR. Ever since I got the truck 5th gear has whined and rattled a bit but I've had other cars do this their whole life and never had any similar problems. Any way, I was about 70 miles from portland, on the freeway in 5th gear and all of a sudden every couple minutes the transmission or clutch would make a noise like I was driving over the little divit bumps on the side of the road to wake up sleepy drivers, then it would go away, then come back, then all of a sudden a horrible vibration and low rumble noise started happening, and when I hit the accelerator it got so much worse, but the truck still accelerated. So I put it in 4th and the vibration and horrid noise went away, but there was still a small faint noise that sounded like a couple rocks or marbles rolling around inside the transmission. It ran like this at 70-75 miles an hour just fine until I got to the stop and go traffic of Portland rush hour. I came to a stop, then started to go again in first, clutch depressed, there is no noise whatsoever, whichever gear is selected, as long as the clutch is in, but I let it out in first and although it engaged and accelerated normally, the faint sound of marbles or rocks turned into what sounded like a whole bunch of giant marbles amplified, second gear the same, third gear a little less, and fourth gear was very faint. I managed to drive it through stop and go traffic, and then when it opened up was able to go 70 again, then it drove through the city and I made it to my destination.

So... The truck still has normal accelerating power in gears 1-4 (didn't want to mess with 5th again, fearing possible total transmission failure), all the gears make the same noise, just at different intensities, and when the truck is sitting in neutral and running, the noise is still pretty bad and gets worse when you rev it up, however, with the clutch depressed all noises go away. Is this a bad clutch component? (who knows how long it's been since it was replaced) Bad pilot bearing? Bad throwout bearing? Broken pressure plate? I find it very hard to believe that all gears went bad at the same time, especially since it accelerates normally. I also find it hard to believe it's a major clutch component since it still engages and disengages just fine.