Noise from rear after 8 3/4 swap

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Sinister.

Sinister.

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So I just finished swapping an 8 3/4 into my Dart. I was pretty excited to get to drive her again :D :D :D Drove around for a while without being to hard and thought I could gear a little bit of a noise coming from out back. It's only there when I'm at 30+ mph. Its not a high pitched whine but more of a lower pitch drone. Seems to get worse as speed increases... I'm wondering if this is exhaust (higher rpm at a given speed) or the rear itself. What do you guys think?

A few additional details:

489 case 3.55 suregrip
727
Mild 318
 
First thing I would do is check the condition of the outer bearings in the axle. If they're good, grease them & adjust the end play. This applies only if you haven't switched to "Green" bearings. I'm also assuming you put lube in the rear. :-D

I'm not sure what you mean by higher rpm at a given speed. If you mean that the noise changes with rpm as the speed stays the same then it's not in the rear axle.
 
I inspected and greased the bearings before reinstalling the axles... end play needs checked..


I mean that with with old 2.76s at 30 mph I was at xxxx rpms. With 3.55s at 30 mph the rpms should be higher which would change the sound of the exhaust.
 
Okay. I guess the easiest way to rule out exhaust would be to try downshifting & see if it changes the sound & go from there.
 
This is easier to do with a stick, but you should be able to confirm/ eliminate rear axle noise by doing a couple of things

First, get up into the "noise" range and a little higher in speed, then kick the trans into neutral, let the engine idle or even shut off the key, and let the car coast. I guess I don't have to tell you to find a deserted street, and make sure the column (if applicable) won't lock

Second thing to do is try to get to a "peak" speed where noise is loudest, and play with the throttle, "feather" it. Especially ring/ pinion noise, and to lesser extent, axle bearing noise, will change as you do this
 
Yep, acceleration and coast, noise changes. If steady noise under both; axle bearings.
 
What I found is that at 55 the noise is fairly loud if I put it in neutral, the noise is still there. Also while in neutral I revved the engine but as I did that, the drone stayed the same. Drone seems to start at 30 mph and when I got to like 45 mph the drone didn't really go away until I was back down to 20 mph.

So this points to the rearend.
 
Sinister. said:
What I found is that at 55 the noise is fairly loud if I put it in neutral, the noise is still there. Also while in neutral I revved the engine but as I did that, the drone stayed the same. Drone seems to start at 30 mph and when I got to like 45 mph the drone didn't really go away until I was back down to 20 mph.

So this points to the rearend.
Click to expand...

If I understand that the noise is fairly steady even in neutral, this points to axle bearings. Ring/ pinion noise generally comes/ goes as you play with the throttle
 
That's correct. At 45-55 mph, in neutral I had the engine from idle, to 4k and the noise did not change.

Ugh. I just got her back on the road too :( ... and as I understand it, changing axle bearings is a pain in the a$$
 
If the bearing adjustment is way loose, the brakes or other pieces could be rubbing. Before pulling axles, check endplay and check brake drums etc for rubbing. Then look at bearing cups and cones before automatically cutting off. Check these for pitting spalling etc to find the problem. If you try to remove races without press, don't pound off w punch. I know from painful experience that pieces will fly off and embed in your body parts.
 
Sinister. said:
What I found is that at 55 the noise is fairly loud if I put it in neutral, the noise is still there. Also while in neutral I revved the engine but as I did that, the drone stayed the same. Drone seems to start at 30 mph and when I got to like 45 mph the drone didn't really go away until I was back down to 20 mph.

So this points to the rearend.
Click to expand...
Hi, Could be pinion bearings too. Hunky
 
When I threw in my 8.75 with a sure grip last year it was doing the same thing, got real loud when I hit the brakes and started to slow down and agin when I accelerated. I found my pinion bearings were pitted. Once that was replaced the sound went away. I still get a little hum, but a few guys with a 3.55 SG told me they have the same sound....go figure.
 
I had a similiar noise situation with my 8.75 and it turned out to be the pinion gear and ring gear starting to eat each other.. I got the chunk used at a great price, and in my excited rush to get the car back on the road we just gave it the once over visual inspection didnt see anything obvious wrong and slapped her together.. in hindsight this probably wasnt a brilliant idea.. lol.. drove it that way for almost 2 summers.. until the noise started getting REALLY loud and noticeable.. needless to say my rear is now being rebuilt by a professional.. lol.. since that was happening I decided to go from 3.23's to 3.55's lol.. should have it back together next week
 
67DartGT340 said:
When I threw in my 8.75 with a sure grip last year it was doing the same thing, got real loud when I hit the brakes and started to slow down and agin when I accelerated. I found my pinion bearings were pitted. Once that was replaced the sound went away. I still get a little hum, but a few guys with a 3.55 SG told me they have the same sound....go figure.
Click to expand...

See this sounds almost opposite of mine. Mine is worse at higher rpms, but as they come down it is much quieter.
 
I see in post #3 you said the end play needs checked. That's where I'd start. If the bearings are real loose cause the end play was not set it can be noisy and will probably start chewing things up. Setting the end play is something that should be done before you ever drive it or your asking for trouble.
 
did it make ANY noise with the 2.76's

i don't think swapping chunks would need much
(if any) adjustment.
...not enough to cause instant axle bearing noise
 
fishy68 said:
I see in post #3 you said the end play needs checked. That's where I'd start. If the bearings are real loose cause the end play was not set it can be noisy and will probably start chewing things up. Setting the end play is something that should be done before you ever drive it or your asking for trouble.
Click to expand...

I kind of eye balled it. What's it supposed to be set at? If anything, the end play would be too tight.

mrbill00 said:
did it make ANY noise with the 2.76's

i don't think swapping chunks would need much
(if any) adjustment.
...not enough to cause instant axle bearing noise
Click to expand...

It didn't with the 2.76s, but that was a 7 1/4. I swapped in an 8 3/4 with 3.55s
 
10-4, i thought you just swapped the chunk and we could
rule out the axle bearings.....
all the bearings are suspect.
..a lift would be good, so as get under it while someone
worked the gas and another could check with stehasope(or screwdriver)\
at each bearing location to locate the offender
 
It has been so friggin cold lately that I haven't been able to do much. But this weekend it warmed up a bit and was AMAZING! So I finally got out and checked end play and it came out to be .020" which is a touch high. Next time I get the chance I'll see about tightening it up, but it seems more and more like bearings.. .002" isn't that much
 
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