T-400 or 12 bolt noise dilemma - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-03-2012, 11:18 AM
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T-400 or 12 bolt noise dilemma

I have a 1940 LaSalle 52 Coupe with a Cadillac 472/T-400 hooked to a GM 12 bolt, all custom fitted. A while back, I rebuilt the rear because I had overcrushed the crush sleeve and had a loud whine in the rear. While in there, I added .005” shim to the pinion shaft to arrive at a total of .035” so as to get a better gear pattern. It worked as best I could tell, the pattern looked better per the charts I looked at. Everything went back together well, new bearings all around. The backlash did not change, it is .010”, measured in a number of places around the ring gear. I used solid spacer(s) to set the proper pinion pre-load at 20 lbs.
My rear end whine got much better, almost none now. But, now I found a brand NEW noise that I cannot locate or understand. At speeds of 40mph+, when on flat ground and I level off the throttle I get a high pitched vibration as I go from load to coast. The noise is not present when under load, nor when coasting; just when going between load and coast. Noise is also not present when in neutral setting still. It sounds almost too high pitched to be internal (inside rear end). I have checked all fuel lines, brake lines, connections, fittings, etc. to eliminate all obvious vibration sources. I was 99% sure I had the issue solved yesterday as I found the tailshaft mounting bolts were completely stripped out allowing the tailshaft to move freely. I oversize tapped this and mounted correctly using the original rubber bushed tailshaft mount, but the noise is still present. I have also put in new MOOG u-joints on both ends of the balanced drive shaft. Drive shaft angles are very near straight at both ends.
The noise sounds more like it is coming from the tailshaft area of the tranny. As it is occurring, I can feel the vibration on the tranny tunnel, though it could be coming from another source. Admittedly, I ran the tranny with almost no fluid while breaking in a new cam – beginner’s mistake that I wish had not happened. But, fluid looks good now and tranny shifts perfect and shows no other bad signs.
Does this sound like it could be a tranny bearing problem? Does it sound like maybe just the change in depth of the ring/pinion could be causing this high-pitched noise?
Any other ideas? How about suggestions on how the isolate a noise like this to get a positive location?
Thanks in advance for the help.

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Old 09-03-2012, 11:59 AM
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rear end noises Ive encountered,small burr on ring or pinion,used gears with new pattern from swapping to a different housing,creak from posi from not having posi additive in oil,axle/bearing,usually a growl. Ive had dry u-joints, tail shaft bearing. sounds like you checked all that so maybe the new gear pattern on the decel side of the gear set?
assuming its not suspension or tire noises. Does the sound persist if the car is run on jack stands?
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Old 09-04-2012, 04:07 AM
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Thanks for the advice Vinniekq2. I have not tried running on stands. I will do that this weekend and post the results.

Thanks
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Old 09-09-2012, 07:30 PM
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I jacked up and ran the car on stands tonight; the noise is there at the same speed range (40+). I used my ear probe thing and it sounds like the noise originates inside the differential. For safety reasons,I did not check anything in front of the spinning wheels. But, when listening to the diff, it is clearly the source. That is good in that it leads me where to go.
If the sound is simply new wear on the decel side of the gears, is it something that may go away with time or will it get worse?
Should I go ahead and put in a new ring and pinion and start over with all new bearings?

I am now at the point I need to do something so I can move on to the next phase of this build.

Thanks in advance for any advice offered,
David
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Old 09-09-2012, 07:46 PM
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If its just the gears whining its more annoying than anything else. I put a used set of 5.13s in my Camaro back when.I drove the car for a couple years with no problems. I tried to reset the depth and back lash with little change in sound.It did subside slightly or I tuned it out with time.If its bearings then you will find out quickly.
If its the main gear set,the sound might change pitch slightly when going from power to deceleration.
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Old 09-09-2012, 07:50 PM
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First try this.Unbolt your driveshaft from the pinion yoke and turn it 180 degrees and bolt it back on. See if that helps or changes anything. Could be driveshaft/phasing problem.
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Old 09-15-2012, 04:49 AM
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RWENUTS - I rotated the drive shaft 180 degrees and the sound remained the same. Thanks for the suggestion.

I plan to take the cover off shortly and look inside. If it is new wear, should I be able to see it? I have out about 50 miles on it since the changes.

Thanks,
David
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