In the steering wheel or the seat? Is it a "vibration" or a "shaking"? What about RPMs?
There's an issue in the 3000-3300 RPM range with the dual-mass flywheel that's standard on the Boxster. I feel it all the time, mostly in the seat. Mine started a few months ago. The problem is similar to a warped rotor, but since the flywheel moves so much faster it's a vibration you feel rather than a shaking like you would get from a brake rotor. The range widens over time and mileage.
Here's what to look for: See if you feel it more when the RPMs are coming down through the 3000-3300 range. You usually don't feel it on the way up because you're going through the range so fast as you accelerate through the gears. As soon as you feel it, hit the clutch and see if it goes away immediately. If it does, it's most likely in the drive train.
Since the Boxster has a dual mass flywheel, I have been told it can't be machined so it must be replaced. The flywheel for the Boxster is $700 for just the part.
To fix it, I will be replacing my flywheel when I do the clutch in a few months with a lightweight aftermarket "racing" flywheel with inserts. Then as the flywheel wears down I can just replace the insert for way less money. That and the lighter weight flywheel will give me more rwhp since it takes less power for the engine to get the flywheel spinning.
