I have a 1999 Forrester “S”, automatic with ~100k miles. This started intermitently but now is consistent. At about 45 mph or higher the rear of the car will shake with increasing intensity. It feels similar to a bad strut that allows the axle to hop. I have checked the balance on all the wheels, bearings feel free, nothing seems to be loose in the suspension, have placed the car on jack stands and run at 60+ with no vibration. My best guess is the carrier bearing or rubber suspension on the drive shaft. It is hard to evaluate in the car.
Any other thoughts?? I would appreciate any helpful ones,.
A Defective Tire Could Do It. I’d Steadily Hold A Pointer Close To The Tread On Each Tire And With Car On Stands, Slowly Rotate Them By Hand While Watching For Lateral And Radial Movement In Relation To The Pointer.
If a tire has any area(s) that move back and forth or in and out in relation to the pointer then you’ve found a tire with a bad belt. That tire is junk. Just to be sure it’s the tire and not a bent rim or axle, hold the pointer steady close to the rim and make sure it runs true.
Before jacking the car up again, you may want to drive very slowly (parking lot speed) and pay close attention. Sometimes with a bad tire you can detect a slight side-to-side wiggle in one end of the car (probably the rear-end in this case).
CSA
I don’t think this is a tire propblem. Speed is too low and it would not be intermittent!
I vote for a bearing or CV joints. But it ought to go into the shop for a proper diagnosis!
I had just written an answer when I re-read your post to see that you already raised the car and rotated the wheels by running the car with no vibration…that sort of rules out the cv joint/axle not running true issue.
Did you have ANY recent work done? Sometimes there are rubber weights on the axles to dampen or tune them, but they dont spin all that fast and dont have the mass to shake your car…only your wheels do… Could it be that your rear wheels are no longer in a straight line going forward? Maybe they are out of allignment? If so that would cause all sorts of issues esp if it were Pigeon Toed…if you will.