OK, so yet another boring driveshaft thread, right? In the process of researching the problem I encountered many different descriptions of the sound resulting from the imbalanced driveshaft. It also occurred to me that although most folks were describing the same problem that descriptions tend to be as unique as the vehicle owners.

Nowadays we can often drive the vehicle and record the sound to give to a repair shop. This is good if your phone will capture the sound especially those sounds that magically disappear when the service manager or tech drives your vehicle. In the event that the sound is too faint for capture or recording we are left attempting to describe the sounds.

For those who have been wrenching most of their lives this is easy. Sounds mechanisms make when there are problems tend to be related to their function. Most automotive NVH (Noise, Vibration, Harshness) abatement engineers work on ways to minimize the intrusion of sound to your driving experience unless it is desirable.

For others, not so much. As a starting point I'll throw a few terms I use out in this message. Please feel free to comment or add your own.

 

Have a look at this: Online Tone Generator - generate pure tones of any frequency

This tone generator allows the frequency and waveform to be set. Be careful with the tone generator and its volume.

For example: Online Tone Generator - generate pure tones of any frequency
The sound the driveshaft imbalance induced was related to it's speed of rotation. The regular motion starts quietly (low amplitude) and increases as the rotation speed increased. To get an idea of what that sounds like select the Sine wave and set the frequency between 120-140Hz. This is what I describe as a hum.

Online Tone Generator - generate pure tones of any frequency

szynalski.com

If the same object is cause something else to vibrate with some contact involved it tends to sound like the same but with that impact noise included. Switch the waveform to Triangle. It is very similar to the sine wave but has an edge, a raspy quality to it. This is what I would describe as a buzz.

Another example: Online Tone Generator - generate pure tones of any frequency
Tire, wheel, or brake imbalance manifests in direct proportion to the rotational speed of the components. To get an idea of how this might sound or feel (below the range of hearing) set the tone generator to about 40Hz with a sine waveform. You can increase the volume (very carefully) to hear/feel this sound. Just don't do it with headphones on. If you insist on using headphones be very, very careful. This is what I would describe as a low frequency vibration in systems like suspension and steering.

Feel free to add description of sounds. Maybe having some reference will help others in the future to describe, find, and fix errant noises their vehicles.

 

Front differential carrier bearings. You will chase other things down but if your driveshaft center support was already done, 95% sure it's your front diff carrier's. Drain the front diff. If it's metallic at all you know where it's coming from.

 

Fluid changes were performed before the driveshaft work. While the diff oil in front and rear was aged there was no visible particulate matter in it. I added a small magnet to each of the lower drain plugs while out. Also changed the xfer case oil in the process.

I should have thought to put some sensors on the under-body to capture the frequency domain around the rear driveshaft. Maybe I'll do that for the front and rear diffs to compare. Might be a fun Arduino project.

If the front diff needs work I may source a lower mileage replacement to build then swap.

 

The front differential made by ZF is always a problem in the touareg. However, since you stated that your diff is good, you maybe experiencing imbalance wheels. It happened to me in the past, one I hit the 75 mph marks, it starts to hump, but if I drop below it, it goes away.

 

I did use an endoscope to look at the gear teeth when the oil was drained. There was no obvious evidence of scratches, spalling, or flaking on the teeth faces. No visible sediment in the bottom of the housing either.

Since the timing chain work my T1 has had about 500 miles put on it. The tires were installed just after the chains were done. Now that I have the major vibration solved a wheel balance and alignment are on the docket. Thanks for the suggestion.