Whirring/Grinding at certain speeds

[green55]

I've got a whirring/grinding noise that seems to be coming from either below the driver seat or maybe behind it. It only happens when I let off the gas after cruising around 55.

I'm not sure if its new, or if I've just quieted all the other rattles and vibrations enough that it just recently became noticeable. I did just replace the rear drive shaft u-joints, which helped with vibrations in general but did not get rid of this noise.

I'm wondering if it's the transfer case due to the driveline angles. I noticed on my slip yoke when I replaced the drive shaft that it does not go nearly as far into the transfer case as it should because of the add a leaf kit I've got on the truck. I'm not sure if that along with bad angles could cause a noise like this.

Any ideas on what it is?

 

[centexk5]

That’s what I have and everything is pointing to the driveline angles coming from the transfer case. Just ordered a Tom woods slip yoke eliminator and cv drive shaft for the rear. For a $1000 it better fix it. Also going to rebuild the t-case while I’m in there.

 

[green55]

$1k...phew. Seems like another reason for me to go back to near stock ride height. Don't want to shell out for new springs/shocks AND a SYE & CV shaft.

Keep me updated on whether that fixes the issue for you. Looks like you have 4" suspension, which I think is more than mine but I honestly don't know where I'm at because I think my add-a-leaf kit was jerry-rigged together.

 

[centexk5]

Didn't have a problem until I switched from 3.73's to 4.88's. Few folks I've spoken to (including the ones here) said the faster shaft speed probably exacerbated an existing issue. The CV joint coupled with a longer shaft should give me some better angles. Gotta pay to play.

 

[ZooMad75]

Having the vibration happen on coast is a pretty good indicator of crappy drive line angles.

4" of lift with stock slip yoke t-case on a Blazer is usually going to have the angles be off enough to cause this.

If you are planning on dropping it down it will help. But I would still take some measurements of the setup now just so you have something to compare to when you put the lower springs on it.

You can get some basic numbers using your phone after you download an angle finder app. You'll want to do this on level pavement. Also set the driveshaft so the yoke on the pinion is vertical, which will make the other yoke at the t-case the same. This will give you a flat spot to measure on.

Take a reading on the bottom of the axle yoke, the shaft itself and the top side of the yoke at the t-case.

You are are looking for a two degree difference between the axle and the shaft and the same at the t-case to the shaft.

You probably have much more that 2 on one side which is the source of the vibration on coast.

Recheck after you install the lower springs, you may still need to make an adjustment.

 

[green55]

I watched this super old but helpful video on how to measure the angles.

I used an app on my phone to measure like you suggested. The angles probably aren't exact but fairly close. Ignore my lack of artistic skills...

Are you saying those operating angles are what should be within 2* of each other but are currently closer to 4*, which is probably what's giving me issues?

 

[ZooMad75]

I watched this super old but helpful video on how to measure the angles.

I used an app on my phone to measure like you suggested. The angles probably aren't exact but fairly close. Ignore my lack of artistic skills...

Are you saying those operating angles are what should be within 2* of each other but are currently closer to 4*, which is probably what's giving me issues?

View attachment 373259

Yeah I wouldn't expect precision from your phone, but what you got puts it in the ball park. Which is to confirm the angles are horrible.

The operating angles are what you want to be within 2° of each other. At the 4.2° amount you are at now it's where your problem is. By running shorter springs you will reduce the angle of the shaft and bring your working angles closer.

 

[green55]

For anyone else that comes along in the future, I also found this Tom Wood's video very helpful in explaining how everything needs to be setup.

To summarize, there's 2 important things:
- Neither operating angle should be over 10*. It will cause vibrations even if the transfer case and pinion angles are equal or nearly equal. Sounds like this needs to be addressed first
- After that's addressed, the transfer case and pinion angles should be as close as possible so they cancel out each other with vibrations.

Looks like I'm breaking both rules. And the >10* rule makes sense in my case because the u-joint at the transfer case is running around 13* and that's the one I've had the most trouble with (it has totally failed with DS falling out).

I'm not sure lowering the suspension will address the operating angles being too far apart because the transfer case and pinion will continue to run at the same angles. The operating angles will stay at 4.2* apart in my case if I'm doing the math right (see below). I think the pinion has to be shimmed to more closely match if needed. However, lowering it will address one of my operating angles being greater than 10, which I have a feeling is the main cause of my problems: chewing up u-joints and the grinding noise I started this thread for.



For the many people that know more than me about this, feel free to correct me!

 

[Wes Harden]

I think Zoomad meant that going to a lower lift spring or stock spring will bring the axle up closer to the frame and reduce the angle at the t case. Dropping your t case by shimming the cross member down will help the t case angle, this is a band aid not a real solution.
Doing a shackle flip will raise the pinion reducing that angle, but you would still have the 13*+ angle at the t case.
A slip yoke eliminator kit will give you a longer drive shaft, reducing the t case angle.
The best that I can think of would be SYE, with cv drive shaft, and turn the pinion up point at the t case out put shaft. probably @ $1k right @centexk5

 

[centexk5]

Yup.

 

[green55]

@Wes Harden i think I was saying the same thing as you, I just didn’t explain it well. Lowering suspension will help with operating angle at T-case. The delta between operating angles will not be changed though, so pinion angle needs to be adjusted (aka point it up more towards the T-case).

Once I finally decide on ride height, I’ll figure out how to get the angles right. Hopefully that’s all the sound is that I’m hearing.

How bad is it to drive it as-is? Is the biggest problem chewing u-joints or am I damaging other stuff?

 

[Wes Harden]

Your pinion bearings and yoke bushing in the t case are under more stress, can't say how much really but there will be accelerated wear.

 

[imiceman44]

I've got a whirring/grinding noise that seems to be coming from either below the driver seat or maybe behind it. It only happens when I let off the gas after cruising around 55.

I'm not sure if its new, or if I've just quieted all the other rattles and vibrations enough that it just recently became noticeable. I did just replace the rear drive shaft u-joints, which helped with vibrations in general but did not get rid of this noise.

I'm wondering if it's the transfer case due to the driveline angles. I noticed on my slip yoke when I replaced the drive shaft that it does not go nearly as far into the transfer case as it should because of the add a leaf kit I've got on the truck. I'm not sure if that along with bad angles could cause a noise like this.

Any ideas on what it is?

Bad angles

 

[green55]

Well, definitely seems like bad angles are the cause. I'll see where I end up after suspension updates and go from there. Glad I noticed this before I committed on ride height because this will be a factor in the decision now.

Thanks y'all!

 

[JoshHefnerX]

I had a problem w/ bad angles. They were setup properly but were simply too acute, and at low speeds it was like driving a paint shaker that would smooth out at speed. Swapping to a cv type shaft cured that.

 

[ZooMad75]

Well, definitely seems like bad angles are the cause. I'll see where I end up after suspension updates and go from there. Glad I noticed this before I committed on ride height because this will be a factor in the decision now.

Thanks y'all!

You are making your way through the math. But continued driving in the current form besides chewing up u-joints you are vibrating the front pinon on the rear axle and the output bushing on the tailshaft of the t-case. At speed it could be a high enough frequency you won't feel much but it's there.

The driveline angles on my '91 were just as bad as yours are now if not worse. Between the shackle flip, 14bff and slip yoke 241 I probably put 25 miles on it before it spit out the u-joint at the axle.

In my case the shackle flip did it's normal thing to rotate the pinion, but the overall size of the 14b actually made the shaft length shorter and compounding the problem.

I ended up shimming the t-case down 1/4" and adding a 6° or 8° shim at the axle to point the pinion to the shaft angle. I then had a driveline shop build me a custom slip yoke cv shaft and did not install a SYE.

While the slip yoke cv isn't as ideal as a SYE and cv shaft to match it has proven to be good over 5 years of driving. Close to 25,000 miles on it and it's smooth all the way to the point the speedo needle can't turn any faster.

 

[Bent77]

@green55 notice what @ZooMad75 is saying about shimming the transmission down. He went 1/4”

A lot of people do a 1” spacer to lower the rear output. This solves some issues at the rear, but then compounds the angle on the front driveshaft. Unfortunately no free meals in the lifted truck world

 

[ZooMad75]

@green55 notice what @ZooMad75 is saying about shimming the transmission down. He went 1/4”

A lot of people do a 1” spacer to lower the rear output. This solves some issues at the rear, but then compounds the angle on the front driveshaft. Unfortunately no free meals in the lifted truck world

Most commonly folks take the bolt spacers for the t-case crossmember down an inch. DO NOT do that. This spacers are to allow the proper clamp load for the t-case crossmember mount. Using sleeves like that to space the case down will put more stress on the bottom of the frame.

Also, like Bent said going that way will effect your front shaft angles in a bad way.

Get the ride height set where you want it. Take new measurements and then start on making a plan to address the problem. Doing anything now is going to change when you put shorter lift springs.

 

[centexk5]

Slight difference in length.