Hum/Vibration only between 30 - 40mph

[spdweb.net]

This is part of another thread I had going but since its a new subject I am going to post it under a new topic. Does that make sense? Anyway..

The original post complete with pretty pictures can be found here:
http://bbs.legacycentral.org/viewtopic.php?f=7&t=51407

The short of it is the transmission had a bad rear input shaft bearing. I replaced both input shaft bearings as well as other various seals and gaskets as well as inspected all the other bearings, syncros and gears. I got everything reassembled and the transmission back in the car. The input shaft is whisper quiet now though what I do notice is a hum or "singing" (similar to tire noise) which only occurs in 3rd, 4th, or 5th gear under light to moderate load, and only between the speeds of 30 - 40mph give or take.

The noise is speed dependent and the pitch/frequency rises as speed increases. It is accompanied by a slight high frequency vibration that can be felt through the center console and a tiny bit in the gear selector. Most pronounced in 4th gear by far. Under light load in 3rd for instance the noise will be slight starting at 30mph, getting louder in 4th and then almost disappearing in 5th until going away completely over 40mph. If I drive spiritedly and hold 3rd until after 40mph, I do not notice any noise at all.

The noise is not LOUD per se, but can be heard in a quiet car. The radio up, or the windows down with highway noise would easily mask it.

So I could almost sum this up by saying there is a hum/vibration in 4th gear under light load but only between 30 - 40mph. There we go.

My thoughts are possible front or center diff.. however.. Usually, or as far as I know, any differential noises are present at all speeds or even louder at higher speeds. I can not pinpoint the noise at all while the car is on jack stands as there is not enough load on the transmission. Another thought was possibly incorrect front diff backlash as I did remove the adjustment rings to install new seals however they were marked and put back to the same spot. I am reluctant to start twisting on the adjustment rings though I suppose I could loosen the lash by one tooth just to see what happens. Another thought was the drive shaft pinion bearing.. but again.. only between 30-40mph? I dunno..

I can only assume the transmission was making this noise before the input shaft bearing replacement but it was just mask by the extremely loud grinding input shaft bearing. The front diff preload/lash is whats worrying me.. as I do not want to be slowly grinding my ring gear to pieces. My gut tells me its something in the transfer case however, until it gets worse I suppose I will just have to live with it.

Well.. that was certainly long winded.. if you need any other details or want more information feel free to ask!

[Legacy777]

I posted this in your other thread, but for posterity's sake, here it is again.

I'm assuming you torqued the main shaft bolt appropriately. Are there any shims or backlash checks on the mainshaft? I don't know if bearing widths change much and if that could affect anything. As you pointed out differential backlash would be my first thought, but if everything was put back together properly then I'm not sure.

It almost sounds like a natural frquency somewhere in the drivetrain. It vibrates/resonates at a certain speed and once you pass that critical speed, everything's good again.

[spdweb.net]

Hey Josh,

Everything was assembled correctly near as I can tell. I don't have any left over pieces anyway. I am sure there are backlash checks on every single part imaginable. I just took it for granted that the new bearings were to the same tolerance as the old ones. I only messed with the input shaft. The bearings on the other shaft were smooth and quiet so I left it alone. Same with the rollers on the front diff, they looked fine so I left them alone also. Though as I said I did replace the seals so I did tamper with the adjustment rings for the front diff. The main nut was torqued to spec.. 86lb +/- and staked in 2 spots. There was some discrepancy in that torque.. but I think that's correct. Though to be quite honest.. since the rear bearing, thrust washer, and 5th gear inner race are all pressed on the shaft, I am not so sure how critical that torque is, as really all it does it hold 5th gear in place. I am sure everything would start to walk around if the nut were not in place.. but 60lb vs 80lb on that nut.. would it really make a difference? I cant really say for sure. Which brings me back to the front diff backlash.. but I would think that would be noisy all the time and more so in a low gear under high load.. not just in a very specific speed range. Also, there is no noise at all under deceleration while in gear.

The only other thing.. The tires on the left side of the car are worn more than the right. Seems the right two were replaced and the lefts were not, dunno why.. bought it that way. They are all 4 matching size tires, but the wear difference would technically make a minute difference in total circumference but is it that critical? I would hope not. The left two will soon be getting replaced so they will be all 4 new and very close to new.

I am willing to try anything that doesn't involve pulling the transmission again. Let me know if you have any ideas. I can certainly live with the noise.. but my mechanical OCD dictates that I MUST know whats causing it lol

[spdweb.net]

ALRIGHTY then... I guess miss matched tire wear IS that big of a deal. I will be getting them replaced tomorrow.

[Legacy777]

Are they different brand tires?

When you get new tires, get the same brand as you already have. You can also check the overall circumference with the car on the ground using a piece of string. The difference between all four tires should not be more than 1/4".