I have a Moser 12bolt with 4.10 gears and an Eaton posi in my '01 T/A. The car also has a Spohn torque arm, Spohn lca's, Spohn panhard rod, and subframe connectors. QA1 coilovers and a Spohn solid rear drag sway bar. Engine is a forged 347 with a TReX cam and patriot heads running through an M6 trans. Here is the issue.

When driving down the highway above 45ish mph, there is an audible whine/howl coming from the back end. If I accelerate and put a load on the drivetrain, the howl dies off some. If I let off the gas or hold a steady speed, the howl gets louder. Also, something in the back of the car has a nasty vibration above 65 mph. It is so bad that it feels like one of those electric seat massagers and it totally blurs the image in the rear view mirror. It feels like something could let go. The vibration really picks up after 65 mph. I know for a fact it is not engine rpm related. It will do it in any gear or with the trans in neutral above 65 mph. Here is the kicker: say I'm at 80 mph. If I ease into the brakes, the vibration goes away completely. It will go away with the drivetrain engaged or coasting with the clutch in. I don't have to get on the brakes hard to make it go away, just a normal level of pressure on them.

I have had the rear wheels checked when I had new tires put on them a few weeks ago. The wheels are straight and true and balanced out fine, according to the local Discount Tire shop. Also, I have a very audible clunk that comes from right behind the transmission area whenever I engage or disengage the clutch and put a load on or take a load off the drivetrain. I have been told by the guys at the local shop here that it is the bushing at the front of the torque arm that is worn and should be replaced with a metal ball joint. Sometimes it will make that clunk if I am driving along and go over a bump, like when the rear suspension and axle move. If I am really smooth with the clutch, it wont make the clunking noise but it is annoying when it does.

Are these issues all related or are they all caused by separate things that have gone wrong? Unbalanced driveshaft? Bent axle? Screwed gears? I want to get this 540hp beast to the drag strip sometime this fall but I am not going to run the car when it feels like something in the back end wants to explode at higher speeds. I appreciate any input and ideas you may have. Thanks!

 

I forgot to mention something else. When I am backing out of a parking spot and I let out the clutch to move backwards, I can feel the rear of the car kinda sink down some and then come right back up when I push the clutch in or start moving forward again. I'm sure it has to be the pinion angle changing, right? Could this also relate to that bad bushing on the front of the torque arm?

 

Check your angle for that vibe issue....

When you reverese the pin drops down, with the suspension style it will cause the car to drop, opposite as you axcel...

As for the whine, you need a slight adjustment or the pin is loose

 

Ok, if it is the pinion/pinion angle, what should the pinion angle be? I'll see if I can get it over to the shop this afternoon and have them check it.

Anyone else have any input?

Cliffs: rear end howls above 45 mph, vibrates really bad above 65 mph, vibration goes away when you touch the brakes. Clunk noise coming from front of torque arm by transmission due to bad bushing, possibly related?

 

The Moser 12 bolt uses four bolts at the torque arm bracket, and these bolts have a tendency to come loose. Loose torque arm mounting bracket bolts will make a clunking sound, sometimes it will clunk more when in turns.
I'd adjust the torque arm for -2 degrees. Bob

 

Make the pin angle 1.5 to 2 degrees lower than the shafts angle...

Check the pinion and see if it has loosened up...

 

I took the car to the shop yesterday and the torque arm bolts at the axle were tight. I know the clunking is from a bad bushing at the front of the torque arm. I have a new del-sphere end on the way from Spohn to replace it.
When the car was up on the lift (4 post) yesterday we checked the pinion angle and it was at -2.5 degrees, which should be in spec. We did a little experiment. We set the pinion angle at 0 degress and I drove it home and to work this morning. There wasn't any real change in the behavior of the issues at all. If anything, the car starts vibrating at a little higher speed now, like 75ish instead of 70. I'm going to take it back this afternoon and put the pinion angle back to where it was since it needs to be there anyway.

 

Check your axles and pinion, was anything loose....

The vibration has a kinda driveline sounding issue to it.... the hitting the brakes stopping it leads me to a few possibilities......

When you hit the brakes a few things happen.... 1. it puts a opposite load on the diff, making the pinion drop.... changing the pinion angle ever so slightly...
2. Also as that happens the pinion is sucked in, so if a noise has a change in pitch and or level...take a look at that nut or bearings...
3. It also holds the rotors, and if a axle bearing was that bad it could calm a bouncing effect at that axle, or if a rotor or wheel has loosened up allowing for a slight wobble...
I had a customer with a vibration, no noise but a vibe that was exactly like a shaft out of bal, right at 65 to 69 mph... after going back to basics (start off with the easiest possibilities) the left rear wheel was ever so loose....