Transmission trouble!

[ryans69chevy]

How do you tell if a transmission is going out? I have a 4 speed manual tranny in a 69 c10 and today when I was driving to college I got about 25 miles and this massive grinding noise started coming. I was going about 63 mph in 4th gear and I grabbed the shifter and tried to move it a lil bit to see if I could mess up the gears or see if it would help and it didn't really do much. The noise would come and go.

After I slowed down and got into 3rd gear going about 15 mph the grinding didn't come back. And then coming home it did the same after about 25 miles...

It felt different than my pilot bearing going out. So I'm confident that it's not that. I replaced that dec 09 along with a new clutch. I've changed the fluid in the tranny once or twice, so I've taken pretty good care of the tranny and I also don't race it or rev up a bunch before I shift so I don't think it's under that much stress. Unless going 70mph and the tach shows 5k was too much for it. I have a 307 with the 4 speed and 3:08's in the back.

Is there anything I can check, or are the hints that I gave a dead giveaway that it is the tranny?