It's a 1998 A4 Avant Quattro with 217,000.

I recently replaced the two worst tires with used ones and had all four balanced. (Budget didn't allow for getting four new tires.)
The right rear wheel bearing was replaced.
The car was aligned.

Now I'm having vibration issues at certain speeds. And it is most definitely speed related, not rpm related.

• When accelerating, between 30-35 mph, it shutters. If I accelerate faster through that window, it still shutters, it just passes a little faster than if I take my time through that window. And it doesn't matter if I'm coasting or accelerating through there, it still shutters. (I generally do my best to avoid cruising between 25-36 mph because it's a PITA...)
  
• It also shutters if I coast down from 50 mph to 45 mph. Does not shutter when climbing to 50, but only when allowing her to coast down through that window. Seems to be the exact same shuttering.
  
• Generally, cruising above 65 mph, she shutters. All the time.

If I try to accelerate while in one of these shuttering windows, it is a bit of a struggle for her to catch on and get going, so I avoid as much as possible. If I can maintain a state of accelerating, it seems to do a little better.

Any ideas??? CV joints?? Wheel balancing? (Some have mentioned road force? What is it?)

Thanks for all the help...

 

What do you mean by "catch on and get going"? Are you saying that the power is affected by whatever is causing the shuddering? Usually something directly related to speed is caused by tires or wheels. If you can arrange it, swap your set of wheels with someone else who has the same bolt pattern (perhaps a Passat). If it still occurs, then maybe inner CV joints, loose steering rack, etc.

 

On a quattro or all wheel drive vehicle or 4X4 you should replace all four tires at the same time...if you have mixed tread/different brand of tires on it and drive it, it will cause more issues down the road and can cause shuddering-but not always....not saying this is your issue now, but it can.

It could be your front suspension, shocks, struts etc...could be a bent rim...a bad tire....if your other two tires are in bad shape, rounded and worn, it will cause this.....If it's speed related....not RPM....meaning if you're going 65mph, turn the car off or put it in neutral and coast and you have these issues....I'd lean more towards tires or rim damage.....just my .02

 

+1 here. When a noise or vibration makes no sense and the wheel bearings are good, it is often a tire with belts that have moved or a weight fell off a wheel. Several times I've diagnosed it by swapping wheels/tires for another set with the same bolt pattern.

 

Thank you for all the suggestions! I have been saving so I can replace the rims and tires with new all the way around. It's just been cost prohibitive lately, so I'm hoping everything stays together in the meantime.

Based on the feedback from the three of you as well as lots of other reading, I am leaning toward it being the tires/wheels. It is currently a completely mismatched set of four tires. They are the original rims and there is evidence of damage. I am inexperienced, though, at how bad it may be. I mean, nothing looks terrible... I think. But...they are a bit dinged up. I should really just post a pic, I guess. (I'll get on that.)

turbo510, you mentioned possible loose steering rack and inner CV joints? Any other signs that would indicate either (or both) of those being possibilities? Like, is there anything else I can look/listen for that would strengthen the case of it being one (or both) of those??

 

Possibly separated belts or bent rim.