Vibration... after getting rear diff rebuilt...

[UH60L]

So, the good news was, after 6 years, I got the rear diff rebuilt and my car back on the road.  The bad news is that after waiting approximately 700 miles, I took it onto the freeway and at 65 mph I had a vibration sound/feeling that was like a constant waahh-unnn-waahh-unnn-wahh-unnn until I slowed down.

Then over a few weeks and driving it here and there and to a couple cruise-ins and car shows, it now does it at 55 mph and sometimes feels worse when I initially let off the gas.

I called Six States, who rebuilt it, and they said it sounded like a driveshaft vibration, suggested bringing in the driveshaft so they could check it.  The driveshaft, which they built for my car many years ago, has only had between 2000 and 3000 miles put on it due to the car sitting so long.  Dropped it off on Tuesday and they just called and said it's straight as an arrow, nothing wrong with it.

So, any thoughts on if it's the rear end or something else and what else might it be?  I know I have read about harmonic balancers coming apart and getting out of balance.  I do sometimes have a squeak/squeal when I start the engine. (never happens when running or driving, only sometimes when it starts...)

My car is a '69 Charger (R/T Clone, originally 318) 440, 727B, manual brakes(front disc converted, rear drum) manual steering, 8 3/4 rear end was the original 2.76, but after the rebuild it's a 3.23 (no posi)

[will]

How old are the tires? There's a thread on moparts about this .

[UH60L]

How old are the tires? There's a thread on moparts about this .

Well... they don't have that many miles, but actual age, they are at least 6 years old, maybe a little older.  Can't remember if I put them on with the new wheels in 2017 or 2016...

Also, there is some wear on the inside edge of the ones that are now on the back. (front end bushings were all shot...)

Maybe they just need rebalancing?...  I definitely can't afford new tires right now...

[Gold Rush]

I don't know if balancing will cure your vibration but if you do go that route, find a shop that does Road Force Balancing. Had a vibration issues several years ago that was cured using that process when typical spin balance did not.

[UH60L]

I put the driveshaft back on and took it to have the wheels and tires balanced.  They said both fronts were an ounce out, so they fixed them, but they said due to the amount of wear on the inside of the backs (previously on the front) they couldn't balance them.

I went and drove the car and it's a little better, nothing at or below 55 mph.  But between 57 and 70-ish it's just as bad or worse, and it tends to get louder/shakier (I can feel it in the driver's seat) when I let off the gas pedal.  It's a constant vibration when I let off the gas, then it settles back to a waahh-unnn-waahh-unnn-wahh-unnn. (almost feels like a sine wave, increas/decrease over and over)

I don't know if all of what it has could be the back tires.  To me it feels like eaither the rear end or something else in the drivetrain...

[doctor4766]

Can you swap a different set of wheels to test that?

[UH60L]

Can you swap a different set of wheels to test that?

Unfortunately, I only own one set of wheels and tires that fits on my Charger, and my current financial situation is, well, not good.  Can't afford to buy a pair of wheels and tires.  I used to be in a MOPAR car club, but it broke up during covid.  Maybe I'll see if someone from the club could loan me two...  Although I won't rule anything out, to me it really feels more like a drivetrain thing.

I read that the pinion angle can cause vibrations.  I did....just before getting the car back on the road...replace the leaf springs.  The set I installed add extra leafs to the right side to counter torque.  Could that have messed up my pinion angle?

[b5blue]

No choice but to swap front to back to see if issue moves. A messed up front end can really hammer tires.   

[Kern Dog]

How old are the tires? There's a thread on moparts about this .

Moparts??

[UH60L]

Sorry for not getting back to you guys for a while, but I got really busy and even started a new job.  Finally got around to driving the car again this spring.  I bought a new set of tires and had them all balanced.  That helped a very small amount but the main vibration was still there.

So I started looking into the pinion angle thing.  I looked like my transmission was pointing downward at approximately 2.2 degrees and my pinion was pointing up at approximately 4 degrees.

If I understand the stuff I've read about pinion angles, at static, sitting on it's tires, I should be downward on the pinion 4 or 5 degrees from parallel (so 2.2 upward minus 4 or 5, so -1.8 or -2., so that it comes up to parallel under load.

I installed 3 degree shims to start and that made a difference.  The vibration didn't really start until I was around 55MPH.

So I pulled those out and put in 6 degree shims.  Again, a big improvement but now the vibration really kicks in around 60 to 65MPH, but I still feel it lightly at 55.  After driving for a few miles I start to feel it lightly at lower speeds until I get under 30MPH.

I've read that I don't want to go much bigger on the shims... but I also don't want to cut of the perches and reweld them (something I'd have to pay a shop to do...).

Any thoughts on pinion angle or if something else could be the culprit?

As mentioned before, I installed new leaf springs before the rear diff was rebuilt.  They are the sportier ones with extra leaves on the right side.  Maybe they are throwing my angle way off...

[green69rt]

Everything you've described reminds me of my situation a few years back on a old van I was driving.  Turned out to be a bad rear U-joint. Easy and cheap to swap out.  A simple check is to get under the car and shake the joint.  ANY movement is a bad sign.

Edit:  Just thinking and came up with another idea or two.  First, look at the front wheel bearings.  Easy check.  Lastly take to a garage and have the rear axles pulled and check the rear axle bearings.  My guess would be your mechanic didn't assemble it correctly.

last edit, check the rear end "trust angle".  Need alignment shop for this.

I keep thinking about things so maybe the REAL last edit.  Have you noticed rear end oil oozing out of the ends behind the brake backing plates?  There is supposed to be a case vent that relieves pressure buildup in the diff.  It's a subtle thing and if you are not careful it can be left off, then pressure build-up will push axle lub past the seals.  In cases like this, I always look at the last thing worked on as the likely problem.

[UH60L]

Well, I noticed during my troubleshooting, that the last few times I started the car, it sounded funny and felt funny.  So, after work yesterday, I grabbed a shop towel and reached down and grabbed the starter, and it was loose and flopping around!

So, tonight after work I'm going to jack the car up and removed and inspect the starter.

Is it possible that some of my vibration was coming from that, maybe the flex plate grinding on the starter?

[UH60L]

Sure enough, got everything cleaned up, installed a new stud (thankfully I had bought a couple of the kits...) and reinstalled the starter.  Car is smooth again, no vibration at all.  Now I just need to un-adjust the pinion angle and make sure it's correct...