low speed vibration, not in the steering

Question

I have some low speed vibration in my car. I feel it more in my butt than feet/hands. It happens while braking or driving. It disappears at higher speeds +25 mph. Because it doesn't affect the steering wheel, I guess its not an alignment issue. Since it goes away at high speed I guess its not a balance issue. Tire shop guy said that it could either be a bad shock or that one of the belts in a rear tire is starting to go. At high speed the wheel expands and stretches out the belts so the vibration goes away. He felt the rear tires and didn't notice anything out of ordinary. He said it might be only slight belt damage. Does this sound right?

UPDATE

1. Its gone on ~1000 miles. I've probably hit a million potholes. I live in Pittsburgh =(
2. I can't really locate the vibration's origin position in/around the vehicle (back/front, driver/passenger). I'll try
3. Its a 2007 VW Jetta 2.5L, with 50k miles.

UPDATE 2:

1. Its not a function of engine speed
2. It is more noticeable at high speed than I initially thought.

So its probably a balance issue. I'll report back after a visit to the tire shop.

Answer 1

I would have commented but this is a little long.

I suspect your vibration's behavior at higher speeds is more along the line of frequency rather than vanishing. I've had numerous of these situations and really it gets down to pinpointing the vibration with some good friend(s) that will go along for the not-so-fun troubleshooting ride.

I would not assume it's in the back just because you don't feel it in the steering wheel. I almost always can find a noise is occuring as well.

Just as an FYI in the past this general complaint on my own (and other people's) vehicles has yielded:

• bad wheel bearing - this is most easily diagnosed by doing some spirited yet long turns. The noisy loaded corner is the culprit if it is a bad bearing.
• deteriorated control arms - This is a tough one depending on whether it's the joint or a bushing. Get creative but for proper diagnosis of a bushing that isn't visually identifiable as compromised you need to have weight on full.
• bushing play - strong rocking and tugs on the suspension could indicate this, but more often when they get bad you will actually see off center wear on cup bushings and pressed type will be broken apart. (I wouldn't necessarily worry about tiny cracks)
• tires cupped - you tire guy would have caught this though.
• rear shock tower bushings - The shocks/struts are held in a bushing system at the top, quite often that center section begins to wobble and it is almost always worse at city speeds than highway speeds in my experience.

There are other odd-ball cases like drum brakes grabbing a little, engine/transmission harmonic vibration that get's "better" with rpms and differential wear.

If you have an automatic transmission there is some credence to transmission mounts but that would be a visually identifiable issue.

Anyway, hope that is of some value to you.