I have a 94 F-150. 4x4, 5 speed, straight 6, 308(?). My question involves the transmission. If I am driving say 40 mph, let up on the accelerator, and coast, I will get a whine/grind noise from the rear. If I put in the clutch it will go away, or accelorate , the noise goes away. The transmission works fine otherwise. It has about 210,000 miles. The noise occurs in all gears.
That sounds more like the rear end than the transmission.
Yup, rearend noise. Given how old the truck is, it may be OK but worn to the point it can no longer be quiet. Have the rearend cover removed, and take a good look inside, check out the gear pattern and look for metal shavings. Put in fresh rearend oil.
And you have the 300 cid straight 6. This engine is nearly bulletproof. Everything else will wear out before this engine does.
I had that problem with a manual shift 1954 Buick. Same symptoms. It turned out that a snap ring broke that held a synchronizer in the transmission. I ignored it and it ultimately did more damage in the transmission. Have a mechanic who is familiar with manual transmissions check it out.