6 speed manual transmission noise?

Hey I am trying to diag a noise coming from my manual 6 speed transmission on an 06 trd off-road. The noise is heard in the cab when driving in gear. It’s a slight hum or grind that could be mistaken for your ac fan being on. When I move the shifter around I can change the pitch in the sound ever so slightly. At low speeds the sound is not so loud. It is loudest in 4th and 6th gear doing highway speeds. 5th gear is completely normal. I just put a new master cylinder in, bled it and changed my transmission oil. The noise was there before this work but has settled down a little after. But is still bugging me. Anyone out there have this problem? Tips or tricks? I’m thinking my synchros are maybe worn down or off. Lmk what you think. Thanks!

 

A hum or a grind? Two distinct different sounds.

 

It’s not really much of a difference in this instance. If I had to pick one or the two I would say hum. And you can feel it through the shifter knob. So obviously deep down it’s a soft grind.

 

If its not grinding shifting into any gears I'd say syncros are working good, I've had gear boxes that had hum in certain gears at certain rpms. What transmission oil did you use and when you drained old out any unusual shavings come out?

 

Does the sound go away when you press the clutch? If so, then it's possible the throw out bearing is going.

 

It’s not a throw out bearing. It only makes noise when it is in gear. And the noise goes away when In neutral or when the clutch is in. Also it does sometimes grind downshifting to 2nd or upshifting to 3rd under load. But I used

$30 75w-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75w-90 gl4 royal purple gear oil. Ever since I installed the new master cylinder and trans oil. The noise has reduced and shifting has become much smoother. But it’s just this noise when in gear driving I can’t seem to understand.

 

Starting to think I might have to just have a transmission expert take a look. It does have 150k on it and if it was driven rough it could just be time for new gears. Also no metal shavings when I did the trans oil. Everything appeared normal.

 

I've had a tacoma that had the hum in certain gears I say just drive it as long as it isn't grinding too often and fluid is good should be able to get another 100k or more just like it is.

 

My 6 speed has always whined in gear while moving. 207K miles.

 

Mine used to whine in 4th, but it has quieted down in the last couple of years.
I use Lucas mostly, but switched to Redline this last time. I'm at about 139,000 miles.

 

What did the master say about your trans?