Phantom rear end rattle under load

[tackhouse]

Hey all -

I've finally got my frame off build road worthy and I am about 100 miles into driving it. I've been experiencing a phantom rattle under load that comes and goes, usually in first gear only - does not happen on the freeway - that I've narrowed down to the driveline/differential. It also seems to go away after a few minutes of driving which seems like it could have something to do with warming up, although I live in a nice climate (Santa Barbara). I can also feather the throttle to make it go away.

It doesn't happen in park. I also cannot replicate it when I jack the rear end in the air and put it in gear.

I took it to a local shop and they think its the differential, so I called the gent that rebuilt it and he came out for a test drive. He's been doing them longer than I've been alive and he said he doesnt think its the internals of the diff. He also said the only time he heard a noise like that it was from pinion angle and there were obvious signs where the contact was happening, which isn't the case with mine.

Everything is new and or rebuilt and I am kind of stumped. Its pretty hard to articulate, but I did my best.

Would love to hear your thoughts!

 

[DirtDonk]

My first thought was pinion angle and driveline vibration. But you usually get it in any gear, not just first.
Any chance you can snap a picture directly from the side, just in front of the rear wheel? Sitting at right of course.
Thanks!
And thanks for being a contributor!

 

[Hallboss]

Check pinion angle for sure. I had similar after just re-installing tranny and replacing worn motor mounts. Also corrected tranny cross member mount. This however changed the T-case, cardan, to 3rd member angle. Vibration was slight, but as I drove it I could feel it in 1st and while making right turns. At first thought it was axle bearings, but pulled both axles, 3rd member and the bearings/diff are solid. Just ordered the 6 degree leafspring wedge from WH. My driveshaft degree of angle definitely increased after the re-install and fixes. Hope the wedges solve the problem for me, if not, time to reposition perch mounts. Good luck @tackhouse .

 

[tackhouse]

My first thought was pinion angle and driveline vibration. But you usually get it in any gear, not just first.
Any chance you can snap a picture directly from the side, just in front of the rear wheel? Sitting at right of course.
Thanks!
And thanks for being a contributor!

I didn't think to put the angle finder on the pinion angle, but here is the driveshaft angle.

Thanks for the replies!

 

[DirtDonk]

It might be the camera angle, but that driveshaft angle sure looks steep! I wish I knew more numbers when it came to optimum angles, but just how much suspension lift are you sporting?
All angles matter. Not just the overall shaft angle, effecting the upper double-cardan joint clearance (you DO have a double-cardan, correct?) but also the pinion angle in relation to the shaft angle.
Your pinion does look just slightly down, but here again, it could be the angle of the camera. Rather than the angle of the dangle.

With a low pinion however, your typical harmonic happens during deceleration. Not acceleration, as you're experiencing. I high pinion angle would vibrate during acceleration, or torque application in an upward direction.
With leaf springs over the axle tubes like we have, the axle twists (known as "spring wrap" or "axle wrap") when torque is applied. Whether going up under acceleration, or down during deceleration, it changes constantly.
If it goes just out of the three u-joint's comfort angle, you get harmonics. Whether you feel them or hear them, depends on which component is doing the resonating. In other words, like so many things on a Bronco... "It depends on whatever."

So while we can't prove your angle is the noise you're hearing, we haven't disproved it yet either.
Apparently, you get to do more detective work. Lucky you!

Paul

 

[Yeller]

The pinion angle is the issue, it has to be within 2 degrees otherwise you will have vibrations. I usually try to get within 1 degree, then if vibration is an issue address the spring wrap as a separate problem that cures the vibration as well.

 

[Hallboss]

It might be the camera angle, but that driveshaft angle sure looks steep!

Agree.... to compare @tackhouse , I have a 5.5" lift and my driveshaft is 21 degrees. the difference with my pinion angle is 7 degrees. Not good for the double cardan. Installing the 6 degree wedges today.

 

[36Fan]

Rattle or vibration? Tin sound or heavier deeper sound?

Put it in gear, apply the brake and slowly bring up the rpms and see if you can hear something. Exhaust hanger, e-brake line, heat shield, look for something with close clearances.