I don't know what this noise is...

So at the beginning of summer I had a noise start happening on my '04 Tacoma. It happens only at highway speeds. when I am giving it fuel there is no noise but when I take my foot off the gas a humming/whining sound happens. It goes away when I give it any gas and it goes away if I coast in neutral. I don't want to throw any potential red-herrings into the diagnosis but I changed out my gearing in my rear differential to 4.88. I did this in 2007 and had no problems with it's performance. I have changed and inspected the oil in my differential, x-fer case and transmission. No more particulate than what would be expected with normal wear. If I move my drive shaft by hand how much rotation should I get back and forth for checking the backlash? Has anyone had any similar issues? Do the symptoms suggest any other problem?

Thanks.

 

In addition: recently I looked at the Tacoma shop manual, I think there is a possibility that the adjusting nut on the ring gear in the deferential could be the culprit. Perhaps it loosened. This would explain increased backlash clearance. What do you think?

 

I found the problem! I took apart my rear-end today to check the tooth pattern and backlash of the ring and pinion mesh. Everything was perfect. I was dismayed because I found no indicators of a problem at all. I decided to put everything back to together figuring the problem was somewhere else. After I put the differential assembly back into the housing I noticed the big locknut that holds the yoke and pinion into place. In normal condition the nut is placed at specific tolerance for bearing pre-load and then it is dented in with a punch into the notch machined out of the threaded end of the pinion. I saw on my pinion nut that it had moved. I had taken my truck into Toyota while back to replace the oil seal behind the yoke and I guess they never re-dented the nut. (This was after I had done my gear job, and I remember denting the hell out of that nut in place). The pinion had a lot of side play to since it was not seated into the taper bearing anymore. I missed this when I checked my backlash because the way I had the assembly in the vice pushed the pinion into place. This would cause symptoms similar to excessive backlash. I tightened the nut and the lined my dent up with the notch and re-dented it. (I don’t have a small torque wrench so I have to wait until I have one to set the preload exactly to where the shop manual says). I put everything back together and good to go. The loud humming sound is history and everything is running smoothly. I’m following the mileage recommendation for breaking in new gears just to make sure everything with my job is okay. As I write this I enjoying a Dogfish Head victory beer.

 

I have a noise just like this, I only hear it when I let off the gas. My truck is automatic though so no numeral coasting. I know nothing about gears what is the nut your speaking of and whats the best way to inspect it? is it possible to fix this with out disassembling the rear end?

My noise sounds like a squeak that I can hear behind me and to the passanger side. It doesn't happen below 10 or 15 mph only above.
Iv done nothing to the rear end since iv owned it but who knows what the po did. Iv been suspecting it's been regeard though.

 

If you were to unbolt the drive shaft at the rear differential, you will see how the yoke (flange that drive shaft couples to) is mounted: there is a 30mm (in my case) lock nut that holds the yoke on to the pinion. Tightening this requires a small inch pound torque wrench. You tighten the nut and then check the drag (bearing pre-load) by turning the shaft with the torque wrench ( dial type is best). For my truck, the resistance should be about 6 inch/pounds (NOT FOOT POUNDS). The only way to do this accurately requires pulling the axles out so that there is nothing affecting the movement as you do this.
If your truck has been re-geared, chances are the gear ratio is different than stock. A quick and dirty way to check this is use a gps and track your speed on the highway, if there is a significant difference between the speed on your speedometer and the gps, chances are the gears are aftermarket. If the speeds are the same, double check to make sure there isn’t some kind of speed calibrator wired into your tachometer. For Toyotas check the transmission, that’s where the speed is read and that’s where the calibrators get wired in. You will have to factor in tire size too if they are not stock size.
As for the noise, mine was a hum in relation to the vibration that was being caused by the pinion not being pre-loaded into its bearing by that nut. I don’t know what to tell you about a squeak. That could be a multitude of things. You need more information to properly trouble shoot. Does the squeak get loader? If so, does it get loader during an increase of speed or rpm? What are the road/ environmental conditions when it happens (i.e. all the time, in the rain, bumpy roads)? Change the oil in suspect components, be sure to drain oil through cheese cloth or some kind of strainer. What kind of particulate does this produce (there will always be some fine metal particulate due to normal wear)? If you isolated it to certain components try and move them by hand and see what you feel. For example, when I had my axles out and drive shaft disconnected I could easily turn my differential by hand, if had felt even the slightest amount of graininess in my bearings (be sure that is what you are feeling, I am a journeyman machinist so I know what’s what as far as this goes) as I was rotating it, I would know there were bigger problems that would require at least a bearing replacement.
Sorry I couldn’t help directly with the problem, but you can’t go wrong with some solid trouble shooting and critical thinking. If you don’t have them, a couple hundred bucks for the Toyota shop manual for your truck is worth every penny.