Technical help needed on a diagnosis

I get a small but very annoying vibration through the steering wheel over 60 Mph all the way up to at leat 140. The wheel shifts left and right minutely. It happens with both my 17" wheels AND my 18" wheels that have both been balanced and have not fixed the issue. I have also had an alignment performed. My local OPC drove it, confirmed the vibration and say they think it needs balancing. Now, the companies who previously did the balance were my local Nashville porsche specialist who I actually trust for most things. The other firm was a more generic tire company who had all brand new equipment (I know this would not necessarily mean quality) Before I have them balanced again I would very much appreciate some suggestions on what else it could be. Before I purchased the car privately I ran a Carfax and it was clean and I had a PPI performed by the OPC where I bought it. I did notice on the final test drive before purchase a small vibration but the owner said it just occured on some surfaces. Yes I know, I'm stupid. Apart from this issue the car is in pretty much mint condition.

Have you had the wheels checked for true? Even if they are balanced they can be out of round. If it is slight you can have it fixed at a specialty shop. Had that problem/fix on my M3

one of the rears was out on the 18" wheels and I got that fixed. I just can't believe its balancing when it affects both sets of wheels

do you have wheel spacers?

When the wheels are put on a balancing machine, they'll know if they aren't balanced. Have bal checked as well as the front end. Any uneven tire wear? Proper air pressure? Did the warranty cover any balance or was it paid out of pocket?

Your tires may be flatspotted. Did you loop it at the track? you might wet the tires (front/back separately) and look at the tracks on a smooth surface..

Wheel bearing(s). I'm willing to bet money on it. As your speed increases they will float and cause a vibration. When you get up to the speed this occurs, turn the car to weight one side or the other and see if there is a decrease/increase in the feel. If this happens, it is a wheel bearing. Smooth road helps. Good luck.

Out of warranty i check my pressures about once a week and they actually never change

Nope

the 18" street wheels have not been tracked by me and the rears are less than 6 months old and appear to be in fine shape for tread wear the fronts are older but they aren't cupped and I don;t think they are flat-spotted - i will check though - thanks for the suggestion

Hmmm when i bought the car the guy said he had one wheel bearing replaced and I have had a suspicion this may be the issue. However when i mentioned it to the OPC they disregarded this completely coming up with some mumbo jumbo I didn;t understand. BUT I am not that impressed with their in-depth technical knowledge turning the car does remove the issue (I think - I will go for a drive now), it is only really apparent on straight level roads. its weird, i've had cars where the wheels need balancing and that vibration mostly goes away at speed, this is only really noticable at 60-70MPH upwards and mainly perceptible on the interstate. I will call my independent. Thank you very much for coming up with this angle of investigation.

I have food in my refrigerator older than the techs at the dealer...

Thats really funny, especially as the guy in question seemed to be not much more that 21

If bearing is bad, you can wobble the wheel when pulling and pushing on it right? My car has a little vibration over 110mph and I'm not sure if it's balance or bearing. I don't want to drive left and right at 110mph!

Chicken! The bearings that are bad can be loose when you push and pull on the wheel; but if this were the case you would know it without having to go to Warp10. You might be able to feel a very, very, very slight amount of play. The forces on a wheel at 110 mph are pretty tremendous.

mine gets progessively worse, the highest I've been is 140 and then I nearly **** myself from the vibration. but around town and under 60 mph there is absolutely nothing

With reference to the play; you might feel it when car is jacked and you grab wheel and move from side to side. How did the drives go?

I have that same problem, I am going to have the wheels balance again to make sure it good. It could also be a slight bent on the rim due to pot holes since the 996 suspension are pretty hard

Did you ever get this resolved? I'm getting a vibration at >80 mph, and I'm wondering if it's bearings. I had the front wheels balanced and the vibration has diminished a lot, but it hasn't gone away entirely.

I'm fighting the same battle with my car (you can see another post from me earlier) - the same feeling kicks in around 70mph and shows up more on some roads than others - below these speeds, the car has no perceptible wobble - but neither does my Chevy Tahoe - I go back to the dealer on Thursday to have my wheels spin balanced again and to have the rims inspected - if that doesn't do it, I may need to have the bearings checked - does anyone know approximately how much that repair costs?

I'm hoping that it's just my tires. This is the longest I've gone with a set of front tires (two years now). I will be changing them early next year. I'll get the bearings checked at my next service interval.