Vibration at any speed.
 

So I bought a 2001 986 Boxster a few months back and I have been dealing with a vibration at all speeds. The sound is very close to a "whoosh" with every rotation of the tire (gets faster and faster with speed). It is pretty noticeable with the top up, not very noticeable with the top down. The whoosh is coming from the rear as their are no vibrations in the steering wheel but a slight vibration in my seat. At highway speeds it sounds a lot like someone driving with offroad tires. The car has 106k on the clock. When I first bought the car, both me and the mechanic that did the PPI failed to notice that the dates and the tread on the rear tires didn't match. Apparently the previous owner got a flat on the tire just 1k miles before I bought the car and he replaced just the one tire. I immediately replaced both rear tires and balanced them. The new tires seemingly made the noise worse, so I got them re-balanced and sure enough, the sound continues. The looming thought in my head is that driving the car on the two mismatched tires messed up the differential pretty bad... What am I missing? Besides this one issue the car is in surprisingly fantastic shape. Almost perfect cosmetically and the PPI revealed really great mechanically. No other issues besides this (knock on wood) and the car has been an absolute dream to own.

Is the wheel true? My tire change guy found 3 tires that wobbled when spun at slow speed by hand on the balancing machine...cast wheels for an Acura.

I would think that any differential damage would manifest itself with a howling or whining noise. Most likely there wouldn't be a great difference in tires after just 1K.

I'm guessing it is tire related. What kind of tires did you purchase?

I would check your wheel bearings. Wheel bearing noise sounds like more of a wump wump and doesn’t go away unless you find a speed that it settles into and somewhat quiets down. Just a guess, but I’ve had cars that made that noise similar to what you are describing.

Busted belt on one of the old front tires. Thats where my money is at.

I got sumitomo z5 tires for the back. Sumitomo z3 in the front.

Wouldn't I feel the vibration in the steering wheel then? The steering wheel is vibration free, or relatively compared to the seat, I feel that the vibrations that are in the steering wheel are just bleeding over from the general vibration in the frame caused by whatever anomaly is going on in the rear.

Have you looked at the transmission mounts and the motor mount?

I had vibration in my car after new tires.
I finally got rid of it by have all my tires road force balanced.

The car for sure needs new motor mounts. It has the vibration at idle in the cabin that you can feel in your seat. I could be completely wrong but I can't imagine motor mounts would cause the noise that i'm hearing. What is the road force balance? When I had my new tires mounted I had them balanced twice to no avail. But just the rears. I figure that the rear is the source of the problem as I don't feel any vibration coming up through the steering wheel.

You didn't mentioned if the vibration is with the engine running (idling) or only when you are moving,
therefore, I would check the engine/transmission mounts and Also the rear wheel bearings..

Road Force balancers are a model of Hunter (I think) tire balancing machines. Discount Tire dealers in my area have them. Tire Rack service centers probably do also.

Road force balancers have a drum that is forced against the tire.
I forget what the pressure is but it simulates the forces of the road on the tire.
So the tire is balanced with a simulated road force on them as if they were on the car and being driven. Cost me $25.00 per tire/wheel but it was worth it.
eliminated all my minor steering wheel vibration through out the speed range.
I have had it up to 143 mph.

As mentioned above, wheel bearings (possible at 100K miles) and tire issues are prime suspects.

Is there any noise/vibration at idle or just when in motion? The reason I ask is the motor mount can cause vibration without shaking the wheel.

How many miles on the car??
Standard trans or auto???

There are two trans mounts I believe they are oil filled.
One center motor mount.
If they are bad it could be the cause of the vibration at idle.
The whoosh whoosh that increases with speed sounds like rotating mass that is out of balance.

Pull your front engine cover.
The one behind the seats.
idle the motor and see if anything rotating like alt. air conditioning pump, anything that rotates is out of balance or loose.

Then look at your axle mounting flanges to the transmission.
There are 6 (I think) torks or allen head bolts make sure they are tight.

Let us know how you go;)

To eliminate possible wheel bearing issues, simply swerve left and right while driving with the noise present. If the sound changes, its wheel bearings. If the sound remains constant, its in the drivetrain or wheels/tires. Hope you resolve the issue

RA1
I reread your first post and see the you have 106 thou. miles on it.
So if your motor mount has never been replaced it is toast. The center rubber part parishes rather quickly on theses cars.

Also if you have a standard transmission.
Push the clutch in at idle and see if the slight vibration changes in any way.
Also take it for a drive and push the clutch in at different speeds and see if there is any change in your whoosh whoosh sound. or any change in the feel of the car.

I will do this thanks. As for checking the engine for loose accessories, the noise acts as a function of vehicle speed, not engine speed. And you're exactly right when you said It sounds like a rotating mass out of balance. I will check the motor mounts and trans mounts. But I'm so afraid its something inside the transmission that it out of balance and needs to be replaced. Smells like something that would hurt the old wallet. I'm an 18 year old straight out of high school working hard to make the Porsche dream a reality. But still praying that it's something stupid that I'm missing.

Usually there will be a "bubble" on the tire sidewall if that is the cause but his vibe is in the rear.

It may be a bent wheel. Any time I am putting tires on unknown wheels, put the bare wheel on the balancer to see how out of balance the bare wheel is, so you have a baseline of what to expect balance wise before you blame the tire.

OK narrowed down to vehicle speed not engine speed;)
That should make it easier to locate.
Drive next to a building, wall, something that sound will bounce off of. With the building, wall, what ever on the right side of the car then on the left side.
See if you can isolate the whoosh whoosh sound to one side or the other.

Also if it is a standard transmission push the clutch in while moving at speed see if the vibration or sound changes.
These cars have a dual mass flywheel when the springs go bad they can go out of balance.
But always go with the easy things first. see if you can isolate the whoosh whoosh sound to one side or the other.

Like others have said it may be as simple as tire balance or a bent wheel.