Odd acoustic resonance above 60 mph in 73 240Z

[pbarcher]

Hi all,

For almost a year I've been trying to troubleshoot an odd acoustic resonance that starts about 60 mph and gets worse as vehicle (not engine) speed increases. By 70 mph it's scary, but below 60 mph there are no symptoms. I'm not referring to exhaust resonance, and it reminds me of the way my Audi sounds when the sunroof is open but the windows are closed.

Initially I suspected the tires, but after replacement with new Michelin's and road force balancing (twice) I replaced the driveshaft, half-shaft u-joints and had everything checked (multiple times) by a local shop. All of the suspension, bushings, bearings, transmission and engine mounts are new. I also changed the urethane stop on my R/T mount after several suggested that the GM type that I originally used might be too tight.

Late last fall the shop determined that the mustache bar had been installed backward, which was causing vibration to be transmitted to the the body. This seems like the solution, so I parked the car for the winter without checking further.

Today was the first outing for the car, and the same symptoms are there. The car has a rebuilt 5 speed transmission and 3.9 R180 differential which were installed at about the same time that these symptoms appeared. However, since I rarely drive at higher speeds I don't know if either could be the source of the problem. It's definitely coming from the rear of the car.

Any ideas?

Peter

[Oiluj]

Can you describe the sound in more detail?

[pbarcher]

It's not a mechanical sound, such as a bearing. Rather it's a low frequency vibration that only appears above 60 and gets worse. Perhaps something in the drivetrain that's out of alignment.

[olzed]

Exhaust bracket or pipe or muffler just touching the body somewhere.???

[240260280z]

If it is not a bearing, tire or vibration.....Is there something hollow and open to the wind? A pipe can sound if the wind is fast enough. We used to hold Coke ™ bottles out the window when we were kids.

The reference to an open sun roof makes me think wind. How are the air bellows that steer wind into the fender ducting?

Try to identify which part of car (front, back, side where the sound is loudest....get a musician with good ears to be a passenger)

[FastWoman]

I remember having a worn lower ball joint in a Datsun 210. Everything would be fine until I reached about 55 mph. Then I'd suddenly get a violent shimmy. A mechanic found the problem after wrestling with my front suspension for about 15 min. As I recall, it took a crowbar to pry the part enough to reveal the play.

[madkaw]

So no vibrations felt or seen at these speeds. Subtle stuff like the rear view mirror getting blurry. I've been chasing a high speed vibration for a long time.

You have eliminated a lot of the variables with tires-good.

Still running the front diff mount + the RT mount, or just the RT mount?

Have you taken a wrench and torqued on every bolt under the car-especially half shaft nuts?

Have you physically grabbed everything that rotates and tried getting movement out of it?

Talk more about the moustache bar-did they relocate it? Was this a Z shop?

Where did you get a r180 with 3.90 gears?

which 5 speed?

[pbarcher]

Blue, it does seem to be wind related. Where are the bellows that you are referring to? The resonance is definitely coming from the rear of the car.

Steve, I'm still running the diff mount + plus the R/T mount and had the snubber trimmed to allow a bit of movement. No change. The five speed is the close ratio model from an 82 ZX rebuilt by Eiji at Datsun Spirit. I found the diff on EBay and had Eiji inspect it and replace the seals.

The mustache bar was relocated with there's no change.

[cygnusx1]

Tape up some body seams and test. Some other ideas...warped or dragging brake drums, worn shocks, misaligned u-joints, bent rim or stub axle, worn rear control arm bushings.

Edited April 7, 2014 by cygnusx1

[pbarcher]

Dave C.

Brake drums, u-joints, control arm bushings and shocks are new. Which body seams do you suggest taping?

[Zed Head]

"Acoustic resonance" seems like a mis-leading term. You have a mechanical vibration, that is noisy, I think. A noise above 70 mph wouldn't be scary, unless it sounded like a ghost. A physical vibration that gets worse above 70 mph, that might be scary.

On the constructive side -

It could be that your rear diff angle is off. With the RT mount, and the mustache bar issue, and use of the stock diff mount (is it new or old? Might need new), maybe you just have a misalignment between transmission and diff.

Your description of wind-related implies a throbbing low-frequency noise, which implies something with some mass. Driveshaft seems to fit.

Edited April 8, 2014 by Zed Head

[240260280z]

Bellows (2) are more like plastic elbows and are up front in the engine compartment where the top rad support meets the fender.