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Noisy manual transmission in SOME gears

94 nissan truck d21, 4x4, v6, manual transmission, 147k miles. Trans issues: Noisy 3rd and 5th gear. 5th is louder than 3rd. 2nd and 4th are quiet. All of these observations are when the gas pedal is depressed/drivetrain loaded. All gears are quiet when the system is unloaded either in gear or coasting in neutral or coasting in gear/clutch in. This noise is about 3 weeks old. All driveline fluids are at the proper levels and look acceptable.

I don’t know if this is related but when in the driveway idling and in neutral, foot off the clutch pedal the trans is pretty noisy too. When I push the clutch in, the noise goes away. this noise is a couple of years old.




It sounds like one of the bearings is going bad in the transmission. It would probably be either the input shaft bearing or the counter shaft bearing on the input side. It is unlikely to be the output shaft pilot bearing or the rear bearing nor the rear counter shaft bearing as these would be noisy in 1st, 2nd and reverse as well. When you are in neutral clutch engaged (pedal up) the input shaft bearing, pilot bearing, and both counter shaft bearings are turning so any bad bearing would make noise. When you push the clutch pedal in all of these bearings stop turning.

It looks like ~$100 for the input bearing and ~$50 for the counter shaft front bearing. You might get an estimate from a transmission rebuild shop for the labor of R&R, disassemble, and reassemble the transmission. You might just let this growl until you are ready to get rid of the truck. Alternately, you could install a used transmission.

Hope this helps.

p.s. On further thinking about this, I am more leaning toward a bad ouput/main shaft pilot bearing. 3rd would definitely load that bearing (upward) while 5th would put a load on it the other way (downward). A bad input shaft bearing or front counter shaft bearing would be noisy in all ratios except 4th which is direct.